This place gets 5 stars because it's in Vegas and there really are very few options for kids in this town. If it were anywhere else it would have been 4 stars.
Went there with my 2 and 1/2 year old on a Monday (they open on Mondays June 1 through Labor Day which is a huge plus). He had fun exploring some of the water things (dams and such) and the big bubble makers. We also had fun turning some cranks on bucket cables and gluing paper and reading in the arts room.
I think the biggest plus was that it was almost empty. I am quite sure that I would not go there on weekends and holidays, but that's not specific to this museum.
They moved the main entrance to the side (behind that cone-shaped structure) so you don't really have to walk through all the library's homeless people (some of which are quite obviously mentally ill). It's still not a pleasant sight, but I'm not sure what they can do about that.
Most activities are intended for kids 4 or older (5 if you have a short guy/girl at home) and there are some cool things for older kids (maybe up to 9 or 10). There is a small baby corner for crawlers and early walkers which is also nice.
I would definitely take my kids there if I were "stranded" in Vegas, but I wouldn't make a trip into town for this.
Really, if it were up to me, I wouldn't bring the kids to sin city in the first place. It wasn't.