Already owning a Tassimo machine, I was attracted to a Keurig because of the wider choice of coffee (and trust me, it IS wide!).
Upon opening the box, I was surprised at the size of the machine. It's quite bigger than the Tassimo, mainly because of the water reservoir being on the side. I felt like that reservoir held less water than it's competitor and after verification it seems to be true. Nevertheless, I like the style of the machine, a great addition to the kitchen counter!
The quality of the coffee varies greatly in the sample pack that comes with the machine. Some types are very good, some are not good at all. Unlike the Tassimo and its bar code reader, every coffee cup is brewed the same way. The lack of milk product, such as in lattes, is also a downside. I'm not ready to spend 90$ on a milk brother for my K-Cups.
Another thing that is quite bothersome is the time the machine takes to warm up water. The Tassimo's doesn't necessitate to turn on the machine to warm up water before you put a cup in it. The coffee comes out much faster consequently.
On the good side, the coffee selection is amazing. While Tassimo only offers Maxwell House and Nabob (not the best around, but good Latte) on top of the much higher quality Starbucks, this machine has over 200 choices of K-Cups. Some of them are truly GREAT and of high quality. While pricier than it's competitor in per cup price, the taste makes up for the difference.
Putting aside the prep time and somewhat small water reservoir, I would say that the Keurig makes the better coffee. The Tassimo machine is better from my point of view, but the end result is what draws me to choose this one as the winner. I'm glad I own both though.