From everything I've been reading, it took quite a while to work most of the non-working feature and compatibility (for most games) problems for the 8500 out. As for performance issues, I'd say both of the major companies in question here usually take a few months after initial release of the hardware before the true performance potential starts to be realized in the drivers. But all of my nVidia cards so far seem to have all the promised features working out of the box, and practically every game works fine right away. Performance wise, there were usually increases later, allowing each card to finally stand out from its predecessor by a noticable margin (GF2Ultra - GF3, for example).
StealthHawk's point about the games that the preview card was tested with is a good one too. Most likely ATI concentrated on those titles first, and then works on other more obscure / less popular titles as time allows. Not an unreasonable strategy, but this means that until the card is in the hands of people that can try it out on more than just the favorite games and benchmarks, we just DON'T really know if they've finally had the big breakthrough in their driver development team that everyone's been crying for, which is that the drivers are basically fully functional and compatibility without problems is high, AT RELEASE. i expect performance to only improve, but I really want to see if the drivers are good all-around out of the box. Nothing worse than having some game that not many people play that I CAN'T because my card / drivers has issues, and that I'd have to wait months before I could..
BTW, I plan to use the same criteria with nV30, especially since it's an all-new architecture for nVidia, and I expect writing the drivers for it won't be quite as trivial as for past cards, which shared a large portion of their core designs (with additions for each generation for the new features).