Pros: Large water reservoir, convenient, easy to use, relatively quiet, ability to use your own grounds.
Cons: Coffee pods are expensive, coffee can be weak.
Before launching into my full review, I'll preface with the fact that I had been using a Senseo single-cup coffee maker for several years. When the Senseo broke, I decided to give the Keurig a shot.
What it is
The Keurig B40 is a single cup coffee maker that brews a cup of coffee in under a minute using K-Cups that come in a variety of flavors and coffee types, including tea and hot chocolate. There's also an optional re-useable filter that allows you to use your own ground coffee, which helps cut down the cost of day-to-day use. (More on the re-usable filter later in the review.) The B40 can brew two different size cups, 7 1/4 oz and 9 1/4 oz. (The larger size is essentially a FULL standard sized coffee mug.)
There's a lot to like with the Keurig. One very nice feature is the large 48 oz water reservoir, which is larger that most competitors and means you don't have to re-fill the tank as often. As with all single-cup coffee makers, the big benefit is the ability to brew a single cup of coffee, which eliminates waste.
The Keurig is very easy to use, with an easy to open cartridge section and easy to remove water tank.
Also, having come from using a Senseo single-cup coffee maker, the Keurig is much quieter by comparison. Some have said the Keurig is a bit noisy, which is true only if comparing it to a regular coffee maker. It's really not that loud at all, especially compared to the Senseo (which sounds like an airplane taking off).
The first complaint can be overcome - the coffee pods are expensive. Keurig uses K-Cups, which are convenient little cups that you just pop into the cup holder. Several manufacturers make K-Cup coffee, tea, and hot chocolate. But, not only is the cost per cup more expensive than some competitors (like Senseo), but you have the added waste of a little plastic cup for each cup of coffee - bad for the environment.
The good news is, unlike Senseo, there are options to use a re-usable filter cup and your own coffee grounds, which eliminates the waste (no plastic cups) and drastically reduces the price. The re-usable filter isn't included, though, so you have to purchase separately. If you decide to go this route, I recommend the Solofill Cup over the Keurig brand "My K-Cup" filter. With the Keurig filter, the water doesn't stay in the cup long enough, so you get weak coffee...and it also makes a bit of a mess, with grounds sometimes leaking out. The Solofill cup is a much better design.
My next complaint is the coffee itself - for my taste, the coffee is a bit weak, particularly if you're making the larger size (a full mug). With the Senseo, if you're going to make a full mug, you would use two of the coffee pods instead of one. This made the coffee the appropriate strength. With the Keurig, however, you use the same K-Cup whether you're making the smaller or larger size. More water, same amount of coffee. To get around this, Keurig suggests using the "extra bold" coffee cups if you're going to make the larger size cups of coffee. I find that odd. If you're going to make more coffee, you should use more grounds. Luckily, with the Solofill cup, you can fill it a bit more - and the design keeps the coffee brewing in the cup just a bit longer, so it's not as watered down.
Since I've mentioned several comparisons to the Senseo, I'll make one more - the biggest difference, in my mind. The Senseo has much more power behind it (which is why it's so much louder), resulting in a frothy crema on top of every mug of coffee. The Keurig is a slower drip, much more like a traditional coffee maker. No froth/crema at all. So, if you're used to a Senseo, don't expect the frothy top if you decide to switch to a Keurig. If, however, you're used to a traditional home automatic drip coffee maker, you should be fine.
Overall, the convenience of making one cup at a time and the ability to use your own coffee grounds make the Keurig a winner. I have one on my desk at work, and have my 2 mugs of coffee every morning.
UPDATE: I thought I'd mention a tip for all Keurig coffee maker owners: they recommend you descale the machine every 3 months or so (depending on how often you use it). I *highly* recomment NOT using the Keurig recommended method of descaling with vinegar. Not only is it difficult to completely get rid of the taste (takes 2-3 full tanks of water), and the smell is aweful, but it does a horrible job of descaling. Instead, I highly recommend using Cleancaf (available on Amazon). It does a much, much, much better job of descaling, doesn't smell, and only takes one tank of water to completely flush out. Your Keurig will be humming along perfectly if you use Cleancaf every 3 months.