I have a XFX XXX 7950 GX2 I received as a birthday gift that performs at close to 7900GTX SLI levels due to it's 570/800 clockspeeds. (compared to 500/600 reference)
I've had no problems with it, and as you can see, it's a powerhouse:
http://www.legitreviews.com/article/354/8/
This card ROCKS- no two ways about it.
I also have a reference model 7950 GX2 which is a bit faster than reference 7900GT SLI.
I've had no problems with either, and am running them in SLI currently. Last night I did an FRAPS on Oblivion during an outdoor battle- 29 minimum, 57 average at 19X12 4X16X High Quality, all settings maxed. I got 64fps average switching HDR for AA and keeping the same settings.
The FEAR test gives me 90fps with minimums in the 40s all maxed 19X12 4X16X High quality.
You could say I've found no reason not to be VERY happy with the GX2s- that much power in one slot is amazing.
You can wait for DX10 cards, but we don't know when DX10 will be out, when DX10 games will be out, what the differences will be in the early DX10 games, and if the DX10 cards will outperform GX2s at <DX9/OGL games that will comprise 99.99999999% for at least another year.
My $.02