The performance is okay for a card that gets released to the public six months later.
I wouldn't see any reason for trading my 8800GTS for a 2900.
If you already own a 8800GTS card I agree there would be no good economic reason to switch. Even as driver changes will most likely push the 2900 further past over the next month, if you already have a GTS it makes more sense to wait till the next level of product hits the street.
That is also why I suggest waiting till mid June or early July for anyone who may be in the market for an upgrade. Around that time good information and other product will be around to evaluate and compare without the hoopla.
AMD/ATi plan to start their more agressive competition plans in Q3