I love Lifetime. This particular club in Summerlin is Diamond level within the Lifetime Fitness company, and it truly is first class. Everything is new, clean and I personally haven't had any issues with children running around (I have been there on weekdays after 5pm).
There is no Lifetime in South Beach and so when I'm in Miami I either run on the beach or go to David Barton Gym (either at the Flamingo or the Gansevoort location) and if the DBG here in Vegas is anything like the ones in South Beach, it is not as good as Lifetime. The DBG in Miami is clean but the lighting is very weird (spotlight over the weight benches, etc.) and it feels like a lounge you can get a cocktail in. The environment doesn't motivate me to work out.
The classes, especially boot camp, are amazing. I can be a bit of a wimp sometimes when working out (I know, surprising right) but the trainers keep me motivated without being too aggressive.
You get what you pay for and although Lifetime is pricey I think it's worth it.