I could name you half a dozen apps which at best slowed down, at worst stopped working after updates.
Didn't realise you could roll back apps via iTunes...will check that out.
There was a problem with iOS 4 which caused things to run really slow on older iPhones, they fixed that pretty quickly. Other than that I've not had nor heard of any issues related to iOS updates - they are pretty stable and have normally been out in beta for many months before hand.
I've never rolled back an app via iTunes so don't know how to do it but I presume you delete the latest version and import the older one. There will be a folder in your iTunes library called "previous mobile applications".
