Had a 4890, really nice card (still working
).
Never had any problems of any kind with it (afaik, nothing major / game breaking).
It was just loud and hot.
Switched to Nvidia recently, physx was an incentive (although you can get limited physx with an AMD card).
Mainly I was just looking to build an ultimate rig to tide me over for the next couple of years +.
(Pretty much everything in the old rig was outdated, I would have Xfired if I could)
Nvidia got their next gen of cards out first *shrugs*.
Also I didn't fancy getting a couple of years in to find I couldn't have a decent dual gpu configuration.
(Current AMD cards seem to have problem with microstuttering in Multi-GPU configs)
Though Nvidia's drivers suck atm. (Crashed 3 times personally within 10 hours of starting it up)
Just google 320.18 WHQL and you'll see why. Hopefully they will be better in the long term.
Not sure which company is better / worse. With AMD backing the next gen of consoles + games, AMD wouldn't be a bad choice. I just couldn't (wouldn't ;) ) wait till ~November.