I don’t think there is a pocket camera on the market that really competes with the SD550 for overall photo quality. The Sony 7Mp 1/1.8 sensor is probably the best they have put in a small camera with the exception of the Fuji 1/1.7. Everything considered, the lens is second to none. It also has an exceptionally strong flash.
But Canon is driven by marketing as is any successful company and the SD550 is just a tad large to be competitive after someone picks up a Casio S600 in a store. So they went with a smaller sensor that has allowed them to add a little zoom range and stabilization while keeping their pocket cameras pocketable.
If the size of the SD550 is appropriate for you I see no reason to be upgrading. If you need something smaller you will likely not find anything as overall competent for corner sharpness.
Canon makes excellent lenses, even for their small cameras. If they couldn’t get great corner sharpness in a small wide angle zoom camera I would guess it is quite difficult. I had a Pentax S4 with corner softness that makes the SD800 corners look sharp as a tack. There was also a little vignetting at wide. I never took a photo with that camera that the corner softness detracted from. Pros in the old days purposely softened and slightly vignetted the corners to give photos impact. There is very rarely anything in the corners you need sharp unless you are copying documents.
There are nearly 8,000 photos here from a camera with very poor corner sharpness. See how many you can find that you think actually affects the intent of the photo.
http://www.pbase.com/cameras/pentax/optio_s5i You would have fewer with the SD800.
The point of the SD800 is the wide angle. For general purpose photography it gives a lot more versatility, both for scenery and limited shooting distances.
I don’t see the warm color in these sample photos comparing them to several other cameras:
http://www.dcresource.com/reviews/ca.../gallery.shtml and
http://www.steves-digicams.com/2006_...0_samples.html In the conclusion of his review Jeff commented that the colors were accurate. Steve also commented that the color saturation was good. One thing I don’t like about the entire SD series is the lack of control over contrast, sharpening and saturation. But Canon usually has a good balance by default.
For artificial lighting it seems that half of Dave’s reviews at Imaging Resource have complaints about indoor WB in the cons. Not only were there no cons about the SD800 in artificial light he commented on the reliable WB in his pros. Steve and Jeff don’t seem to test that as much but they had no negative comments. With poor colors and WB problems you might have gotten a bad camera.
