I've given away 6 Keurig brewers as gifts. There were 1 B-100, 3 B-50s, 1 B-40 and 1 B-60. At home I have a very nice espresso machine, so I don't have a Keurig myself.
The Keurig web site will show the differences between the models. They all brew the same way. The fancier models let you chose whether you put 6 or 8 oz of water through the k-cup so you can get a richer/fuller bodied cup with the B-50. Maybe it is the B-60.
The machines are convenient. Everyone one I have given away is still in use daily. I'm told by my friends/family that it is the most often used kitchen appliance.
I've tried the Senseo machines, Flavia, Black and Decker, Home Cafe with Folgers. Keurig has the best tasting coffee and is the best brewer. It beats pods hands down. The Tassimo machine is interesting, but much more expensive and there isn't as much choice in coffees.
If you want a work horse for the home to make a predictable good cup of coffee with a minimum of fuss, Keurig is a great option. If you have people with different schedules or different tastes, it is great.
The two downsides are with the extra bold coffee, sometimes the coffee drips from the k-cup if you don't turn it sideways as you take it out of the brewer (a sponge or a little training and this isn't a big deal).
The other thing is that right now, the best way to get k-cups is through the mail and on the web (unless you pilfer them from the office). It is hard to match the amount of coffee to order with actual consuption. Guests, parties, family visits etc. can easily throw that out of wack.
OK, three things. I've been drinking espresso straight up for a few years. Some k-cups can taste thin. But then so does a lot of brewed coffee, so that isn't the fault of the brewer. It is one of consumers' tastes and design specifications of the k-cups.
On a scale of 1 to 10, I give Keurig a solid 8 for home use. In the office, it is a 10 just because of all the nuisance it avoids. No burned coffee. No cold coffee. Etc.