I have been thinking about buying powershot 3 to use a backup to my 30 d .What is your opinon of this camera .

Are you talking about the S3IS? I have given this camera some thought myself, but that's as far as I got with it. On paper, it looks like a pretty good camera, but I have never used it. A coworker has it and he really likes it. The biggest turn off for me is that tt doesn't take CF cards. This isn't a big deal tho, since the price of memory has fallen quite a bit in the last year.

What do you plan to use this camera for? I was thinking about getting it as a light weight, walk around camera for hiking, and biking. That would be my camera of choice if I didn't want to carry a lot of gear, but I want to invest my $ in some better lenses for my SLR's before I get another camera.

I bought an S2 to compliment my 10D. I used to have a G2 in that roll. The S2 is very similar to the S3 and I find it to be a decent camera with some frustrations. The big zoom range with image stabilization is the feature that sold this camera to me. The movie mode is great and actually quite fun.
Likes:
Big zoom
IS
Movies

Dislikes:
Plastic body
Focus is spotty
Weak flash-no hot shoe

Overall I like the camera, but miss my G2 a little.