Klub Ujezd was the first night club in the Czech Republic. This allows it some liberties that few other places can claim -- for example, their famous street festival every year, where the club and all its dreadlocked madfolk (and all their out-of-town cousins) spill out of the bar onto the sidewalk, dressed in costume, partying into the night. Although this may not be so extreme, keep in mind that street parties are rarely allowed in prague, and all outdoor activities must cease by 10pm.
The interior is fantastic. The iron work is the best that can be seen anywhere, the product of one of the masters of the craft. The bar staff are friendly without being artificially so. The basement is a hole of sin, which you should venture into only if your need for oxygen is not high. The top floor, my favorite, has a couple of rooms and plenty of tables for you and your friends.
Rarely have I gone to Ujezd and not met someone new, be it Ivan the giant black-clad manic depressive, or some Cuban traveller. If you haven't been to this bar/club, go.