i rode rigid to start riding on a beater mongoose for about 2 years, then picked up a FS and rode it for a couple years, and then built up a 26er HT SS. once i got on the HT SS set up, i loved it - it was nice and light and the SS was simple. the fork was toast on the thing, and i wanted a new ride (just got the bike bug) and so i went looking, and now have a HT geared 29er (a niner mcr).
i think i found the ride that makes me happy right now. its comfortable, i feel real in control of the bike, and the best part is i feel a like the bike doesn't lose as much energy as my old FS (and i need all the help i can get in that dept). i may not have been losing energy on the FS, but i felt it mentally, especially climbing, and sometimes all it takes is the brain to convince you of something - real or not. once i had the SS, climbing became a whole new game for me versus something i just had to do to go downhill.
but i think one's liking to one's ride changes as one continues to ride. you start looking for different stuff to do, and you look for the ride that makes you most comfortable on that 'new' stuff you're trying. so i'm always looking for a new ride, its just when i get the bug enough to trade/swap/purchase a new ride, i'll go for it. but right now i'm feeling great on my HT 29er, so no new trail rides for me (but i am now looking for a HT urban/dj bike for some off trail fun - thats my new sickness - i f'ing love bikes).