Honestly, the only major change is the 2.5" (vs 2") LCD display. This display actually makes the battery life a hair worse, and it's already somewhat worse than average for this class of camera. Other than that, the SD500 has been great. We've had one since earlier this year. Get an extra battery though, that's for sure, and a big SD card (make it a high speed card in case you decide to use the movie mode).

The specs show that the 2.5" display also makes the SD550 slightly thicker than the SD500. There are some other minor changes noted in this article:

http://www.dcresource.com/reviews/ca...ew/index.shtml