Well, having heard releases at Photokina---the expectation was that the camera would be released at the upper end of the APS-C market (Canon 7D). So one could imagine a $2500 MSRP at launch (or maybe $2999 and $2500 street). At that level, the IQ would dominate the stuff currently out from Canon, Pentax and Nikon (at APS-C), and one could envision Sigma actually beginning to build market share.
There are 'issues' with competing in that market segment. No video, no live view, no unquenchable buffer to shoot 300 frames at once. But the IQ would trounce. And the IQ might give some of the higher end full frames a run for the money (optimistically, the SD1 might be better in that area). No ISO 128000, not nearly as 'robust' as a D3x or Canon 1D Mk 73 (or whatever they're up to). But low ISO IQ---solid.
When you kick this up to a professional level, you need to have a professional customer support system. You should have tethering (the SD1 doesn't). There's a lot of stuff a pro is going to want that the SD1 isn't going to have. The IQ? That'll be there. I'll even hazard a guess that the lens selection is good enough for most now (50, 85, 70-200, 120-300, 50-500, maybe 8-16), and it's likely to get better. But there's not the infrastructure in place at Sigma to sell this as a pro camera.
I hope that I get a chance to test the camera. From what I've heard from my friends who have had a hands on, the body feels very good, we're back to two wheels on the top for shutter/aperture, the LCD on the top isn't missed because the LCD on the back does the job, AF is better, and ISO 100 is exceptionally clean in later firmware/software builds. I think Sigma will have a winning camera, if they can price it 'reasonably'.