There must be some variables at work that are creating iOS 5.1 probs for some and not others. My iPad 1 and 2, and iPhone 4S, have been working fine since I upgraded to iOS 5.1.
On iPad 1 and 2, things have actually been better than fine, because I've barely had any RAM-related crashes on either iPad, which I was frequently having on iOS 5.0.1 and 5. For me, this upgrade has been a big improvement.