Firstly, I have an Ati 5770 from Dec 09, bought for its DX11 abilities back then.
My 5770 experience has been better than the 4850 experience, in fact the 5770 has been very good. However, overall I see Ati as the Ford Xr3,i a kinda poor mans choice, the NVidia is BMW/Mercedes (for those who are willing to pay more for the extra performance and quality). There are indeed times when maybe Ati/Ford step ahead, but on balance the more expensive produce always is the better choice (Nvidia/Merc and BMW) for those with deeper pockets willing to pay that extra few £££ for the extra quality/performance.
As with most things, you get what you pay for is usually the case, always buy the best you can afford is not a bad way to purchase things, rather than what 'appears' to be the cheapest