The 5800 is a flop. To many things went wrong and too many things are still wrong.
If it had come out before the R300 it would have kicked ass, it's still faster than a GF4 and is still fast enough to cope with next gen stuff. But it just doesn't have what it takes, IMO.
Too much hype and way too much technology. IMO nvidia decided that they could push the barrier forward without waiting for DXx.x and believed that everyone would follow.
But as ATi has shown, just sticking to the current spec is the best way to go (F-buffer apart - can't see where that fits in yet!).
Nvidia bit off more than they could chew, realised it wasn't going to work out like they wanted, looked for a excuse and then made the sad mistake of believing their own marketing machine.
Oh well, there's always next time.