SSD = Solid State Drive
Recommend this product?
HDD = Hard Disk Drive
I am writing a very long intro because you can more fully appreciate the quality of a drive like this with it. I detailed my user experiences near the middle and end of this review.
Hard disks work by magnetically storing data (zeroes and ones) onto fast-moving platters. A read/write head assembly, which looks like the arm of a record player, can change the magnetic fields. For instance, you can overwrite a photograph with a new version, or save an email attachment to your hard disk.
A big problem with HDDs is that it takes time for the read/write head to reach the portion of the hard drive that you need to access. It only gets worse if you want to do multiple things at the same time such as save an email attachment while ovewriting a photo while downloading the latest version of a program.
Another problem with HDDs is that they reach maximum speeds of 70 MB/s on slower hard drives, and high-end HDDs capable of sustained 100+ MB/s speeds are more expensive.
Further, files may get fragmented over different places on different platters, so to read the same file the read/write head may have to travel to a bunch of places scattered all over the place, which further delays things.
Lastly, all of that platter-spinning takes energy, so hard drives use up several watts of electricity, which costs electricity and generates heat. Even though HDDs are resistant to heat up to 45C, that waste heat can impact the reliability of other components like your CPU and GPU. (This is usually not a problem in a well-ventilated case, even with several hard drives, but I've seen some really badly ventilated cases before.)
Enter the SSD. Many people are familiar with flash memory. It's the kind of solid-state, non-spinning memory found in cell phones, memory cards (for your camera, camcorder, etc.). Flash memory tends to be slow and top out at about 64GB in consumer memory cards, but if you put enough flash memory together, they can work in parallel to increase speed and capacity.
In theory you could use a typical USB flash memory card as a hard drive, especially if it's a large-capacity drive capable of housing an operating system and programs. The earliest SSDs were pretty much just a bunch of smaller flash memory pieces glued together. There are many problems with simple SSD designs, though:
First, flash memory has limited lifespan. HDDs may be slow and all, but those platters will live for decades--long after they are obsolete. For typical consumer-grade flash memory, each cell can read/write a few thousand times before failing, and a typical daily load may eat about 5-10 GB due to write amplification. Therefore it is absolutely critical to use higher-grade NAND that can withstand more wear and tear than a flash memory card is expected to withstand.
Second, SSD memory controllers must be more robust than flash memory cards. In the event of a power surge, power outage, or other problem, you want your drive to not corrupt data. If it corrupts data, such as mangling your Windows 7 installation, you may find files unreadable yet think they are perfectly fine until you actually read them. If a HDD fails, the data is still on the drive and may be recovered, albeit at a high cost by professionals. But if a SSD fails, it takes all of the data with it. (So use a good power supply unit if you use an SSD. If you have a pre-built computer by companies like Dell, Apple, and HP, don't worry too much about this, because they typically use decent PSUs made by companies like Delta. What you really want to avoid are the no-name "free" power supplies that come with el-cheapo cases like Raidmax. As a rule of thumb, the heavier your PSU is, the better quality it is, because more and higher-end parts tend to weigh more.)
Third, SSD memory controllers should make some effort to coordinate flash memory. If done properly, SSDs can have read/write times much faster than HDDs, with higher throughput, and no fragmentation problems (there is no hard drive platter to spin, so you are limited to the speed of an electron through silicon circuits, which approaches the speed of light). If not done properly, the flash memory won't work in parallel as well.
Fourth, due to limited lifetimes of NAND, good SSD controllers will try to spread the wear-and-tear out among more pieces of flash memory rather than hit the same portions of the SSD over and over again. Also, overwriting portions of NAND is slower if there is data already there, so controllers can cheat--for a while--by telling the user that no data is at X location after deletion of a file, and then actually going in and cleaning up the garbage later. This wear-leveling and "garbage collection" is automatically activated with Windows 7 via TRIM, but you will need to activate it with anything older than Windows 7. (Google for how.) If you are using Mac or Linux you will be okay.
There are only a handful of major flash memory makers in the world left today after industry consolidation. The big players include Samsung, Toshiba/Sandisk (joint venture), and Intel/Micron (joint venture). Samsung makes its own SSDs. Toshiba doesn't make consumer SSDs, but Sandisk does. Intel and Micron both make SSDs for consumers as well, though Micron brands theirs "Crucial." There are also some smaller players like Hynix (which doesn't directly sell to individual consumers except via some Corsair products; they like to deal with businesses instead), but Samsung/Toshiba/Sandisk/Intel/Micron make up the majority of the market.
These flash memory makers also sell their excess inventory and lower-quality flash to companies like OCZ, Corsair, Patriot, Kingston, and others. By lower quality, I mean flash that has imperfections, perhaps due to contaminants in the environment at the time they were made, so that the flash has to be run at slower speeds, or has worse durability.
Micron doesn't buy flash from others, so it can keep the best for itself to use in drives like the Crucial M4, which is the first plus. The only other consumer SSDs that can say the same are the Samsung, Intel, and Sandisk SSDs, but the Sandisk SSD uses a SandForce controller with known bugs. The newer Intel SSDs also use SandForce controllers, but Intel fixed the bugs after spending literally a year on the problem.
The second plus is the Marvell firmware that has only had one hiccup so far, unlike other companies who have had all sorts of issues with their controllers and firmware, including corrupted data, entire drives dying, etc. If you buy an M4, go to the Crucial website and download the latest firmware and upgrade to it, and you are good to go. (If you buy a M4 in August 2012 or later you are probably fine, but there's always the chance that you buy an old M4 that was sitting on the shelf for a year or more.) I've had my Crucial M4 128GB for almost a year and it has behaved flawlessly.
The third plus is that the M4 performs pretty well. It's a middle-of-the-pack 6Gbps SSD, meaning it is compatible with SATA-III standards (backwards compatible to SATA-II if you have an old motherboard; heck it's even compatible with SATA-I though if you are still on SATA-I it's probably time for a whole-system upgrade) and will be difficult to discern its performance from a Samsung 830 or Intel 3- or 5-series SSD.
Note that Marvell doesn't use a fancy compression algorithm like SandForce controllers, so your Crucial M4 performs the same on highly compressible files (e.g., typical OS files) as on incompressible files (e.g., video files, JPEG photos, etc.).
I used to play Left 4 Dead 2 on my Steam account installed on a fast 500GB server-grade 7200rpm HDD, but it still took up to half a minute to load a level. After installing the M4, I am almost always the first to load and enter the new level. Windows loads much faster (less than 20 seconds, and the system is responsive the moment you log on, as opposed to taking almost a minute just to boot up and then showing you the desktop and behaving very sluggishly, ignoring your mouse clicks), so rebooting after a software update is no longer the "ugh do I have to" experience it used to be. Even websurfing feels faster. There is almost no lag time between when I click to open MS Word and when it loads. Etc.
The fourth plus is that the M4 is usually among the cheapest of the most-reliable SSDs (meaning Samsung, Intel, and Crucial; and arguably Plextor as well because they tend to use high-quality components and trusted controllers). I don't know if it's because Micron is a USA company and has lower shipping costs or what, but Intel drives using the same flash (Intel and Micron jointly operate flash memory fabrication plants) almost always cost more. You can go even cheaper but then you dip into the world of SandForce bugs and lower-quality NAND (or both). SandForce has gotten better, but for ultimate reliability, Samsung/Intel/Crucial are still tops. For instance, the SanDisk Extreme has been plagued with problems and for months after launch it still didn't have functional TRIM.
The only drawback to the M4 is that you need to have a little free space for it to do its garbage collection properly, so don't fill up the drive to max capacity. Typically what people do is they install their OS (e.g., Windows 7) onto the SSD, plus their most-often-used programs (e.g., web browser, office suite), and maybe some games if they have space. The rest, like photos and videos and music files, are best left to cheaper storage drives like massive 3 terabyte HDDs which are much cheaper on a per-megabyte basis and which won't suffer too much by being left on a big slow HDD. (For instance, even the slowest modern hard drives are fast enough to stream multiple HD 1080p movies unless there is a bottleneck somewhere like USB 2.0 or 10/100 Ethernet; get USB 3.0 or Gigabit Ethernet to widen those bottlenecks.)
Now that SSD prices have fallen to under $1/gigabyte, it's become affordable for a lot more people to fly on a SSD. Just make sure you get a reliable one and you'll be fine; data is too precious to entrust to sketchy SSDs. My primary desktop at home runs the SSD I'm reviewing (the Crucial M4 128GB SSD) and hasn't skipped a beat, ever.