The Neon Museum is legit. One of the coolest things I've done in Vegas outside of a casino or pool.
First thing to note is that you will need to make a reservation several days, if not weeks in advance. The spots fill up quickly and I would be shocked if you were ever able to just walk up for a tour.
Secondly, in the spring slash summer slash early fall, it is hot as a mutha out there. The sun is just beating down on you the whole time. There is no shade and the umbrellas they give you only do so much.
My advice is to try to go early in the day or at night. When I say early I mean like 8am early. The tour is an hour long. We went at 8:30 and by the end we were dying.
Everything else is amazing. The signs are totally awesome. The stories behind them are even better.
The cost is around $19 for a daytime tour and I think it's like $25 or so for a nighttime tour.
Not bad considering your purchase is going towards the refurbishing of the signs and keeping the museum afloat.
A must-see for any trip!