I've switched from 21 speed mountain to FG/SS then to 10 speed geared. Not counting my project bikes since I've only ride them for "tests" until they're complete.

How'd it go?

Mountain to FG/SS: Had to get used to the geometry of track style bikes and skinnier tires. However the bike was much lighter and more responsive and I felt quicker (once I got the gearing I liked). Watched a TON of track cycling to learn efficient standing starts which help with my acceleration and it definitely smoothed out my pedal stroke and increased my comfortable range of cadences. I feel obligated to go fast to keep a comfortable cadence.

FG/SS to Vintage touring: Alright I guess. It helped me learn a lot about bikes and building them. Probably don't feel as comfortable because of the cassette gearing (5 speed cassette and double ring font). Definitely feel slower and less responsive, but hey, it's a touring bike. I feel it's easier to go slower when given gear change options.