If ATI could get their act together and produce good drivers from the get-go and not 6 months or a year later, then it would definitely be a company to contend with. The hardware is superior to Nvidia's but the driver support has not been the greatest. Sure it works great on your machine, but many people have had problems in different programs and stability. Try that with Nvidia's cards and you will see that overall there are alot less people complaining that they have problems getting set up and working(and this is strange considering Nvidia has a much larger market share than ATI so you would expect more complaints).
Now I am going to get the 8500 cause hey, it's a great deal and I don't mind playing around with drivers, plus they're more mature now, but if I had to have cutting edge and both the GF4 or R300 came out today, I would go with the GF4. I would like to support Ati more cause they're a great Canadian company but I shouldn't have to wait months while they work out the bugs on a product that is a generation old. Yes, the 8500 is $100 less right now, but it's been out for months and the GF4 has just come out. I'm not guaranteed that Ati will support the 8500 after they release new gens of their cards(not universal drivers) but at least with Nvidia the drivers can be used way back to the TNT, and since this is the case, I'm going to make sure that the card works now before I buy it otherwise it might never work they way that it should.