On it's on merits, the R520 is a very nice card; however when you compare it to the G70 and factor in the time that has passed since the G70, it's a failure in my eyes. Why? Although the R520 beats the G70 in 3DMark2005, it's only equal (at best) with the G70 in real world tests.
The R520 is faster than the G70 in some D3D games, but the G70 is faster than the R520 in all Opengl games save perhaps the COD 2 demo which is an anomaly it seems. Anyway the G70 is as fast and sometimes faster in some D3D games as well. Keep in mind that this is at default clock speeds for both cards. The G70 is giving up nearly 200MHz with it's core and 300MHz with it's memory! ATi has said the R520 can O/C quite well. The G70 can do the same too!
By all accounts the R520 should utterly dominate the G70 in everything, but it doesn't! So what's left? Possibly the IQ, ATi's Avivo tech, the 512-bit Ring Bus and the 512MB of GDDR3 it has. Still, looking at screenshots from both cards and trying to see which has better IQ is difficult. What that says to me that even if the IQ is superior on the R520, you shouldn't have to sit there and stare at a two pics for 5 minutes to see that. If that's the case (and it has been for me) then it doesn't matter because if it takes time to see just a little difference in still shots, how are you going to possibly see it at 60, 80 or 100 FPS? Then there's the price and availability!
Had the R520 come out at the same time as the G70, and the price was the same then I would probably declare the R520 slighter better. Even if the price drops rapidly to match the G70, nVidia is still the champ. People say, but the R520 is faster in 3D05...well so what?!? If it can't match that in the real world, it means nothing! The most important scores are real world scores. If synthetic benchmarks support that then it's a slam-dunk. The R520 only wins clearly on 3D05 but changes drastically with real world tests.
The bottom line is the R520 is too little, too late! I'd like to see how the R520 would fare against a G70 with the same core and memory speeds. I do think clock for clock, the R520 is actually more efficient. After all it does only have 16 pipes compared to 24 with the G70. If the R520 had 24 pipes though, there would be no debate whatsoever as it would dominate the G70 in all D3D and probably all Opengl games as well. I think perhaps the R580 will do this. If there is a 32 pipe version of the R580 then oh my...that would be truly amazing.
Still I can't imagine ATi releasing a card that powerful in the Spring of 2006. Most if not all Spring cards are refresh cards of the previous fall. I think the R580 will probably be a 20 pipe card with only slightly raised core and memory speeds. That would definitely be faster than the R520, yet not give everything ATi has. I think ATi will hold back until the R600 and then release 24 and 32 pipe cards. The R600 will be the first ATi card that will have full Windows Vista functionality I believe. If ATi can do with the R520 architecture as they did with the R300 architecture, the R600 will probably be the most impressive card since the 9700 Pro! I think the R580 could be that before the R600 but I think ATi will hold back because like I said the Spring usually brings refresh cards and ATi will save the best for the Fall. Let's hope they don't pull what they did this year.
Don't count nVidia out either as it's track record for coming back from 2nd place is legendary. I'm not saying nVidia is in 2nd place now. Some reviewers have said that with the release of the R520. I'm saying the R580 has the potential to blow nVidia away and I think they know that. Rest assured they'll have something planned to combat the R580. I think nV is looking toward Fall of 2006 though and I think they'll surprise everyone....much like they did with the 6800 series!