I have a mix and match of several machines to play with (about 20) all are 3-reel. A couple have the big progressive display on top and a couple have the "buy-in" option, all pretty much work the same though. The lowest payback percentage is around 84, and the highest is 93, and playing them, you can definitely see the percentage in action. Since they're all earmarked for man-caves, I think the highest percentage would be the most fun to play. And oddly, most of the game chips - M1-2-3 - are the same chips. A few of the games have the P2 chip installed, mostly the progressive and buy-in machines. Pretty decent variety between them, enough to get me into trouble mixing up the insides....
On the progressive displays, I'm trying to figure out what to do with them to make them more interesting. They use little solenoids for each of 7 segments on each digit, very odd that they'd do it that way, sort of like a billboard sign. Apparently I can program that display to increment a certain amount each time a coin is played, and each win.
Out of all those machines, I have maybe two sound boards within them, and only one of them actually works. Jackpot bells are missing on some of them, so I have my work cut out to make them all work. Fun Fun.