I wont write much as its all being said above,
I went for Sema 2010 and loved it! Hoping to go back for Sema 2012. I stayed at the Excalibur, cheap, cheerful and the room was actually pretty good, it was at one end of the strip, next to the MGM grand, close to the monorail (station is in the MGM)
Night life is good, all depends how much £££ you have to spend, either meals and shows or party hard in the clubs.
Top of the World restaurant in at the top of the Stratosphere is expensive but worth a visit.
I get excited just thinking about going back haha