How does it work? Is it slow or does it work just fine? It seems to run like iOS 7 on my wives iPhone 4S but I have not yet done the upgrade to iOS 8 on mine.
I can't speak for 8.1, but the previous iterations of 8 left my 4S nearly inoperable for anything other than the phone. Safari would crash and die with almost every web site. The whole experience was extremely sluggish as well.