I use the Natural. I bought a Sculpt, but took it back because of the top row of keys being so small, and a couple of other gripes. I bought another Natural, but I always ponder the Kinesis line. Besides the unknown experience, the price is also a factor.
My cube-mate has a natural and I use a kinesis. We both thing the keys on the kinesis have noticily better action, but he just can't (doesn't want to) wrap his head around the grid layout. Unless you have very small hands, or you just absolutely hammer on your keyboard, it would be unlikely for the kinesis to make your thumbs sore. The space button on the ms naturl is junk compared to cherry mx brown switches.
