In my teens, I had a Schwinn 10-speed (at the time, Ford was in office); since then, I've progressed steadily -- 12-speed in the 80's, 18 speeds, in the late 90's, 24-speed 5-8 years ago, and 9-speed since then. Tried a 21 for a short time, went quickly to 24 on that bike.
6 and 7 speed rear always left me looking for the 'right' gear; the 8-speed rear worked well. I went to 9 because I wanted to try the 'new' SRAM X-series. The result of that? The only Shimano I'll ever run again is XTR brake cables (and maybe FD). SRAM XX 10-speed? Yeah, right, the cassette costs more than my entire X.9 drivetrain! (While I wouldn't mind X.0 shifters, the X.9 does it for me overall! Best system I've EVER used!)