What everybody says is true. This museum isn't shiny and polished, but good luck finding a wider array of machines spanning the history of pinball. it is a nonprofit entity that will provide hours of entertainment if you love pinball. If you're not a huge pinball fan, try the old-fashioned quarter pins of the 60s and 70s. They're simple and charming, and not as demanding as the machines of the 80s and beyond.
I spend a few hours there every time I visit Vegas, and I spend about $10 per visit. Best entertainment bargain in Vegas.
Back in April they had some of their machines near the food court at Riviera. I'll find out if they're still there when I visit next week.