I have been twice and still enjoyed my second visit. Through bad planning, I ended up going with a group of guests right after hitting the Mob Museum. Even after three hours at the previous museum reading a bunch of placards, this stop was still a hit and the agreed upon favorite. It helps that my teenage nephews are all interested in science and were sufficiently awed by the power of the bombs discussed.
The staff is obviously very enthusiastic about the subject matter and we all loved the stories of casinos offering tour buses out to watch the testing back when they were above ground and the dangers were still unknown. It is a little chilling watching people intentionally stand in a radioactive fallout zone, but one of the guides told us people still come to the museum that were there back then. So obviously they didn't all grow a second head or turn into a radioactive superhero.
The down side to this museum is that it could use a little updating and maybe some more interactive features. If you don't have the patience to do the reading, you may not get much out of it. The AREA 51 Exhibit was still being worked on, so we missed that though I imagine that will be a good addition.