A friend introduced me to C'est What? about 5 years back, and it became my instant favourite bar in the city. If you've been to Bier Markt, C'est What is the same...but totaly different. Whereas Bier Markt offers beer from pretty much every country in the world, C'est What is all local micro-brews, all the time. Steamwhistle is probably the only major label brand they carry, and even that is brewed just down the street.
Staff are super knowledgeable, so don't be intimidated. If it's your first time with micro-brews and don't know what to order, just tell them what you normally drink and they can bring you something comparable. If you want to try something different, they are happy to make suggestions. I've seen this time and time again - these guys know their stuff.
Oh, and the food is great too. Excuse me, I hear a pint of Dead Elephant calling my name.