I absolutely love Canon, which is why ever since I had the A620, i've stayed with them. I now have the Canon S3-IS and I love it!
As I just said, I used to have the A620 which is basically exactly the same as the A640, but with only a 7.1MP resolution. There were a few things I didnt like about it, one of them was the blur you sometimes got, especially when zoomed up. Another was a delay between when I pressed the shutter and when it actually took the picture.
The S3-IS that I have now has the flip-screen, which is useful to prevent dirt from getting on it, and scratches, but it also has the 12x optical zoom and optical image stabilization.
I can say that the image stabilization is a huge bonus, it helps out alot when taking pictures (especially when zoomed up or when your taking pictures in low-light situations without a flash (higher shutter rate = easier blur). Also, the image stabilizer is also active in the movie mode, which helps out alot as well.
I'll post some pics in a minute of some photos I took with my Canon A620.