If you are Las Vegan then you owe it to yourself to check out this attraction! You will get an excellent discount and it is probably the most cost effective and educational way to entertain yourself for an afternoon in Las Vegas!
The Mob Museum has everything you can ask for in a museum. This one is exceptional in that the building itself is a part of the attraction, which I think adds to the attraction. It also has a lot of awesome artifacts and interactive exhibits. My personal favorite was the recreation of the Kefauver hearings which took place in that very courthouse.
The parking around the museum is tricky and it will cost you a couple of bucks, which is lame. However, you may as well take advantage of the fact that you are parked and walk over to the mob bar and check out the real "old" Vegas.