Osman,
I think I posted to you earlier, but considering that it's the same sensor in a very similar body---it's an entirely better camera.
1. Improved buffer (21 shot) means that I'm never waiting for the thing to clear (buffer clearing is similar to the SD14).
2. Way better battery life. I'm getting 700-900 shots per battery, when there were times where I'd just get 100-200.
3. Better LCD
4. Better controls (ISO button, QS menu).
5. Better continuous focusing mode. AF might be slightly better (but not huge---I'm expecting the SD1 to be much better).
6. Much better colors from 'auto' settings, including white balance. It's easier to get stuff right in SPP than previous versions of the camera.
7. Color modes (which I don't use much). In fact, I leave the camera in Neutral because the histogram most closely matches the 'raw' data (IMO). The other color modes seem to cook the high end by about 2/3 of a stop, and that might cause you to reduce exposure if you have the camera set to a difference color mode than neutral (the histogram on the back is a processed jpg according to those settings).
8. No lockups. My SD14 would lock occasionally. The 'generic' fix is to update firmware and not use the auto-review on the LCD (and not fill the buffer). The SD15 hasn't locked up for me, even when I do abuse the buffer in testing.
The only bad that I know of so far with the SD15 is that the Sigma flashes don't work with the TTL mode on the camera. The 530 version issues can be fixed with a firmware upgrade to the camera (which is not yet publicly available). The 500 version needs a firmware upgrade to the flash as well, so you'd have to send it in. I use my flashes off camera with cybersyncs, so it's unlikely that I'll go through the trouble.