My impressions.
1. Battery life to die for (because the batteries don't die). When we were out for a 2-day shoot with no charging, I was really worried about my SD14, because it eats batteries like no ones business. The SD15 was great in that regard, much less hungry, with a good battery meter that worked well. In fact, I took one 'blinking' battery out of my SD14, put it into the SD15, and got an additional 200+ shots out of it. Frankly, I was very pleasantly surprised.
2. Great LCD on the back. I will say that because of lockups with my SD14, I've turned off the auto-review on that camera. The SD15's display is bigger, much easier to see, faster, and with better info screens. I didn't use it as much as I should have, because I've trained myself not to... But I was quite impressed and happy.
3. Speed. Faster in many aspects. This wasn't critical to me because of the subjects we were shooting (I never pegged my SD14's buffer in 2 weeks), but I did fire a bunch (10+) of rapid fire shots to see if I could. And I could... Also the image coming up to the LCD seemed faster to me, but I didn't time anything. I didn't notice any major difference in AF speed, although I didn't use autofocus much
4. No lockups. In all the shooting (I had about 1200 files), I didn't have any issues with the camera locking up. It took dust storms pretty well! All files were good.
5. IQ. For me, it looked good. My only issues with IQ on the SD14 have been in that 'underexposed' areas of black dogs in white snow, or underexposed shots inside. We didn't shoot anything resembling that. We were outside with plenty of light, and the camera did a great job. There was a tendency to underexpose shots because of the 'apparent' brightness of the LCD image, and there may have been some calibration differences in the two cameras that I shot. We reported those issues and I've been told that they're fixed in shipping units.
6. 8-16? Loved it. I've got a really good 10-20, so I don't need this lens. I think I'll need the 85 f1.8 when it comes out, so I'm probably not buying it myself. But I'd definitely recommend it as a wide angle choice. I love the way it handles flare—it's beautiful, while the 10-20 is pretty ugly.