The only (that's "only") adverse impact I see is that my 8 no longer delivers mail or message notifications on the lock screen, and searching the Apple support forums it seems that quite a few other people have had the same problem. My 7 is still fine. I wrote "only" because that may not matter to a lot of people. For me, it's almost mission-critical. YMMV.
Everything else seems fine - speed, battery life, etc.