Pretty cool spot... Great ambiance and generally good music too. It's a nice place for locals and those that want a change of pace from the typical bar scene or the nightclubs located on the strip.
While I overall would recommend Commonwealth, there are two complaints I have. First, for being a bar that will primarily attract locals, the prices on drinks are highly comparable in price to the inflated prices at nightclubs. Not a lot of bang for your buck either, in my experience. Second, it gets insanely packed, again you feel at times that you are in a nightclub.
Overall, Commonwealth is definitely worth checking out, even if it is slightly overpriced and crowded. The ambiance and general feel of the bar is pretty much unlike anything else in the city, and the DJ's they have spinning definitely add to that feel.