Great museum. We spent 3 hours there and could have stayed even longer, but it was getting late and we needed to get to dinner.
The exhibits are all very well done, but they are very text-heavy, so be prepared for that. It was a slow day when we were there- we only encountered three or four other groups of people, but it still sucks when you get stuck behind a slow reader!
The exhibits are self-guided, with lots of photos and plaques and several short movies that play on a loop. You start on the third floor and work your way down, from early waves of Italian and Irish immigration, to the increase in gang activity during Prohibition, all the way through to present-day. There are exhibits at the end about Whitey Bulger, so really, I mean it when I say present day!
Great way to spend an afternoon in Vegas. Get out of the heat, away from the crowds and the smoke, and learn some fascinating history.