This is supposedly the oldest brewery in Europe, or something like that. They have been brewing the same type of beer since the 13th or 14th century. Bottom line is that this place has been around for a long time and is worth visiting both because of the beer and because of the history behind it.
Because of its status as the "oldest brewery," it is very touristy. They have huge long tables set up and people running around serving steins of beer. The beer is dark, but very good. People will often break into song and you'll end up singing with other groups of people there.
They made us take shots of Becherovka with our beer. They don't exactly explain what it is very clearly, but forced it upon us. All they say is that it "aids digestion." Becherovka is a Czech alcohol that began as a digestive aid and is very herb based. It is made in Karlovy Vary (aka Carlsbad) and is actually pretty tasty. I recommend at least trying one shot while at U Fleku.
Make a stop at U Fleku, soak in the scene, drink a beer or two, and have a shot of Becherovka. You will understand why you just need to do this once.