I've been using iOS7 on the iPhone 4S for over a week now, it's pretty buggy. But no where near as buggy as iOS 6 beta 1, which was unusable.
Apps crash a lot, facebook and other apps crash if you click on some links
Its slow/sluggish as hell.
Sometimes when your switching between apps/closing apps via the multi tasking list - when you select to go back to the home screen all icons disappear until you hit the lock button and then swipe to unlock.
messaging app crashes if you try to take a picture and send it to the contact you are texting/messaging. choosing an image from your library works fine thou.
My car struggles to connect to the phone via usb cable and play songs sometimes it screws up the radio in the car completely until I disconnect the phone
I have the above problem when I connect via bluetooth for hands free calling.
I've not notice to many things, a part from random crashes and the phone reboots its self from time to time. Made issue is speed for me and the animations laggy and things like that.
I also hate how when you tap in to a folder it zooms the background wall paper too, its annoying and distracting.
There is probably other bugs I've forgotten, there's been that many. But it is much more stable and usable than iOS 6 beta 1.