Usually i'm a Phantomskinz fan, but since we are talking about just the back I have to give the nod to Best Skins Ever as well. I just finished installing mine, and it looks great. Out of all the iPad 2 "self healing" skins BSE did the corners the best. It's also has a great mix of smoothness and tackiness that makes it a perfect back skin.
As mentioned, BSE is probably the cheapest of the bunch, but this doesn't mean they are lower quality. IMO BSE is able to price their skins cheaper because they cut out the "fluff"... There is no fancy packaging, solution, crappy cloth or application card. All you get is the skins and your packing slip. Their instructions are online and most everything else you need can/should be already in your home.