Pros: Inexpensive, simple and reliable, easy to use, works great.
Cons: Some people may not like lack of extra buttons.
I go through a lot of mice. For some reason, and for as long as I can remember, I've held my mouse different from most people. I hold it with my hand straight, but the mouse at a diagonal, pointing up and to the right on my mouse pad. This leaves me to hit the left mouse button with my middle finger, scroll the mouse wheel with the middle of my ring finger and hit the right button with the bottom of my ring finger. While this may be odd, I've grown accustomed to it and it works well for me. Unfortunately I tend to somehow wear out my right mouse button a little quicker than most, and I have to get a new mouse button because it quits clicking correctly.
Also due to the way I hold my mouse, an ergonomic mouse is practically unusable to me, as are smaller travel sized mice. The same holds true for many of the newer mice with extra buttons, or goofy trim pieces everywhere. On top of that, I just plain hate the looks of many of those flashy mice. I saw a review the other day of a mouse shaped like a car, and while that was awesome as heck, I wouldn't (even if I was able to) use it myself.
PS2, USB, Wireless or Bluetooth?
The wireless and Bluetooth mice both have similar pros and cons. I wanted a wired mouse because I've had such terrible luck with the battery life in wireless mice in the past. In addition to this, wireless and Bluetooth mice can experience occasional signal loss and latency. As for a PS2 mouse, while my current computer does have a PS2 port in the back, another computer in the house does not. PS2 mice have already been all but replaced by USB, and soon they will be phased out entirely in the same way serial mice were phased out by PS2. That left me with needing a USB mouse.
Ball, Optical or Laser?
Even if I could still find one for sale, I sure wasn't going with a ball mouse that would skip and stutter and required constant cleaning to function. I also chose not to get a laser mouse. They may be a little more precise, but they also cost more and the little bit of added precision was unnecessary for me. An optical mouse it is then, and the Logitech M100 Mouse features high definition optical tracking at 1000 dpi (dots per inch). This makes for smooth and responsive pointer control that is pretty responsive and doesn't jitter or jump around the screen. It's more than adequate for all my computer needs, including my extensive gaming sessions. It's comfortable in my hand and tracks well on my mouse pad as well on the 6 foot heavy plastic folding table I use as a computer desk.
The Logitech M100 Mouse is a full sized mouse that can be used either left or right-handed due to it's ambidextrous design. It's approximately 4.5 inches long, 2.5 inches wide, and 1.5 inches tall. It's a pretty light-weight mouse, and the cord is about 6 foot long to allow plenty of room to position the mouse precisely where you need it. It's grey in color with a black mouse wheel and trim. The wheel scrolls smoothly with a light clicking noise each notch, and can also be clicked to the side to allow side-to-side scrolling or instant photo zooming. All the buttons click smoothly with a light but firm click. The Logitech M100 Mouse is just your average, three button (2 + wheel) USB mouse without any particularly distinguishing characteristics or features, which is exactly what I wanted.
PC: Windows 7/Vista/XP
Mac: Mac OS X 10.4 or newer
Linux: Linux kernel 2.6 or newer
All: USB port
The package only includes the mouse itself and a thin quick-start guide. No driver software is needed -- just plug it into an available USB port and you're good to go. It does come with a three year limited hardware warranty from Logitech, but it's such an inexpensive mouse in the first place that it would probably not be worth sending back when or if it eventually required replacing.
Personally I just buy a new mouse every year anyway, but this one seems to be holding up better than most and maybe I can skip a year this time. It's nice to have the quality and reliability that comes with Logitech mice. I really like this mouse myself because of the ridiculous way I hold mice, but I'm also pretty happy with it in general. There are no real downsides for anyone who doesn't require the extra buttons or ergonomic features of more expensive mice, or extra precise tracking of a laser mouse. Wireless mice are really nice themselves, but between battery life and signal latency they're not a great choice for anyone who plays any twitchy games like first person shooters or people (like me) who use their computer an excessive amount.