My iPhone 4 is still on 4.0.1
Jailbroken, Unlocked and running along smooth as silk.
I don't care about HDR photos because I already had an app for it, and I'm not worried about find my iPhone.
The only problem I've had is that I can't get the 8mm camera app I want without 4.2.1 (which I don't understand why that is) and I can't facetime with a Mac.
I haven't seen a reason to upgrade and lose my JB or the extra $$ the unlock will bring me when I sell it this summer for a new phone