Maitrea near Old Town Square
I stumbled upon Maitrea quite by mistake. My girlfriend and I had been wandering around the area looking for some kind of place to have dinner. We'd actually thought about going to a steakhouse the same night, but down a different street, Maitrea seemed to be a more unique restaurant with pretty decent prices (for the area).
Our first visit there was wonderful. The food was spot-on, and the service was quite good as well. I have been back a few times since, and the service is generally good. Sometimes there's a bit of a wait, but I suppose that's understandable considering how busy the restaurant often gets. The food has been almost always delicious. There have been a couple menu items that I'd not order again, though.
All in all, however, I'd suggest Maitrea as a solid place to eat if you're looking for quality vegetarian food in the Old Town area. If you're willing to cast a little bit wider of a net, though, I'd encourage a stop in at Radost FX near I.P. Pavlova as well.