It depends on how big "big" is. They can do amazing things but some dents, too big, too deep, poor access, etc. they can't work with. There's no reason not to have someone look at it though and give you an estimate. If they can do it's much better than getting out the bondo and paint gun. I prefer the mobile guys to the fixed overhead operator - they seem to be a little cheaper but still do good work. Ask around and you should be able to find a good one.