It's a next generation card. Of course it's going to look good compared to what's out now. But considering it has double the theoretical power of the 4890, with more than double the transistors (and a much bigger die), seeing only 50% more performance isn't very impressive. If Nvidia doubles up the performance of the 285 (which is possible given a 512-bit bus) then it won't be pretty.
Look at it this way: had the 2900 XT launched 3 months before the 8800 GTX, we would have seen the some of the same reactions, concerning the performance increase over the X1950 XTX and 7900 GTX.
Was the 2900 XT a bad card? No. Did it deliver the expected performance increase over the previous generation? No. The 5870 is like 2009's 2900 XT: both doubled up transistor counts, both introduced a new version of DirectX, and both seem to offer a similar performance boost.
Of course if Nvidia fail to execute, it may be enough. There may not be an 8800 GTX this time around. But it doesn't mean I have to be impressed. The transition from the 3870 to the 4870 was impressive. This isn't.