I was considering getting a skin to cover the back of my iPad, but I decided against it because of the price and because I know I won't like the feel of it once it's on and will probably end up removing it (as I've done with the other skins i've applied on my iPhones in the past).
I currently only use the grey polyurethane cover, and I didn't buy a case at launch because almost none were available, and now I want to wait until I find one I know I'll really enjoy before investing in one.
So for the most part, I only use the smart cover. I do put the I
iPad in a burlap sleeve that's meant for my MacBook whenever I carry it around in my Backpack. I try not to put it on a table if the back is going to make contact with the table, because I don't want to scratch the back. Usually I'll just place it face-down so that the smart cover is what's in contact with the table. Or if I'm using it, I'll fold the smart cover behind the iPad and set it on the table that way. When I was at home though, my parents moved my iPad around, and I found that they placed it on a table with the back exposed, or I handed it to them to show them something and they ended up placing it roughly on the table. It makes me cringe, but I have yet to see a scratch on the iPad.
When I prop the iPad up with the Smart Cover to type, I do allow it to make contact with the table, but I place it carefully. I don't have scratches yet, but I do treat it very carefully, and it is a hassle. So I'd suggest getting some sort of back cover. I'm just holding off to find something I'll like.