I had warned my son about upgrading his 4s to iOS 8, but he went ahead and did it anyway. Now he's seeing some delays in some of his operations, but he downloaded the SwipeFree keyboard app to try out. I have the update waiting on my Mac, but I haven't gone ahead with it yet. Still waiting to see the "I shouldn't have updated" articles.
I'm not using a 4S but everything is running smoothly on my iPhone 5 and iPad 4 with iOS 8. I haven't noticed any incompatible apps or crashing. The only hitch I have seen so far is that Safari doesn't feel as good on my iPad. It's not broken but they have changed things a little bit. The main thing is that I can no longer just hit the X in the top right to close a web page. I have to hit the button that brings up the multi-window view first even if I am only using 1 Safari window. I'm sure I'll get used to it eventually.
Also when I do this it pops open my bookmarks along the left side of the screen and there is a slight hitch every time it does this. That didn't happen with Safari on iOS 7 for me. Everything else feels pretty much the same. My apps don't feel any faster or slower and everything I have used so far still works in iOS 8.