This is hands down one of the best museums in Las Vegas! I did an afternoon tour yesterday (they're all guided tours) and our tour guide, Manny, was SO entertaining and informative! Seriously, I learned so much about art, neon, the history of Las Vegas... All kinds of stuff. I definitely recommend it and can't wait to go back.
FYI, some of the guides told us they like the day tour better because you can see more of the sign detail and color during the day, but that the night tour is cool too because some of the signs actually light up. I'm planning to go back at night sometime (the tour was that good) so I can see both.
Make sure you buy your tickets in advance because it sells out fast!