I only ever knew one person in who owned an atari st back in the day. We'd often do the amiga vs. st thing at school, but it was mostly in good fun as he was pretty willing to admit he'd have preferred an amiga. Even though he was pretty honest about the differences in the machines he swore that Super Hang On was faster on the ST, which is something I should check one day.
Ironically I never played any ST software until I ran StonX on amithlon/os3.9 about 6 or 7 years ago. Was fun to see what I was "competeing" against all those years ago.
After being used to the amiga versions of much of the software I looked at you really "feel" the use of the amigas custom chips. Even things like sound (which incidently is awful) could sometimes slow things down onscreen on the ST vs. Amiga. Games that were little more than ST ports in particular are a good showcase of this as when static they look the same.
Anyway, it was interesting to check out what the "other side" was using back in the day, but it mostly re-enforced I'd made the right choice back then :-)