There is radon, by the way, in all natural products. You are hundreds of times more likely to be at risk for radon emanating from the soil beneath your home. For a good read on radon in granite, try this:
Granite and Radon and this:
snopes.com: Granite Countertops and Radon
One recommendation I happened upon when researching granite was to be careful that your granite does not come from a third world country or Russia.
As in most things, there are different qualities of granite. the top two quality stones will not water stain. There is a 15-year sealer that can be applied ($300 for our Begonia countertops). We've never had a staining or any other problem with our Level III granite from Ultimate Granite.
Up north, we had Level I granite. It was sealed normally, and any water left standing on it would cause a dark stain that lasted till it dried out. Not permanent.
You are supposed to be able to put a hot pan on it, but I don't. One granite company up north said theoretically you can do that, but on a rare occasion you could hit some kind of seam that could cause the granite to crack.
Our corian countertops in this new house were a scratched-up mess within a year. Yes you can sand them down, but the scratches tend to occur in the same places where you use the counter a lot. So after a while, you'll have an indentation from continued sanding. I found I couldn't even put a UPS box down on it without protecting the counter. It was a nuisance.
Look at soapstone as well. More expensive than granite, but totally impervious. Some beautiful blacks with green or white veining. Martha Stewart has them. The slabs are not as wide as granite slabs, so you might end up with a seam in a designer kitchen.
