I have a Canon SD450 with the AW-DC50 All-Weather case. I love it, and I've taken a lot of great pictures underwater with it, especially on the Great Barrier Reef and on some shipwrecks in South East Queensland. I got the camera with the case rather than one of the new waterproofs because I really wanted a Canon, and they currently do not have any waterproof models (that I know of).
Even at $115, I think it was a great buy and I certainly took much clearer pictures (and plenty of movies!) with it. The only two problems I have had are 1) it is an all-weather case, and therefore only waterproof down to 10 feet. This is good for rain and snorkeling, but when I get my scuba license I would like to have one that can go much further down, but unfortunately they don't make actual scuba cases for the SD450. Newer models like the SD600 have scuba cases though. Problem number 2 is that my O-ring (the waterproof seal) has already worn out, and I've had the case for 9 months, and have used it 5 or 6 times in those 9 months. I took the proper precautions--greased it with silcon gel, made sure not to stretch it each time, cleaned it thoroughly afterwards--yet on the trip I took last week, it was so stretched that it would not properly seal anymore, and on testing it without the camera inside in the water, it let small amounts of water in that would have been enough to adversely effect my camera. So, I guess the moral is, always have a spare O-ring!
Here's a picture I've taken with this camera/case combo: