We recently upgraded from the iPhone 3GS to the iPhone 4S. In general it has been a good experience, however we aren't sure how to turn off 3G and go to the EDGE network on the 4S. It doesn't appear to be an iOS 5 thing since older phones with iOS 5 still retain the ability to turn off 3G and switch to edge. Instead of turning off 3G being an option (which would kick us back to edge) we can only turn off cellular data, but calls don't revert networks.
This is very important when using the iPhone in our area since the 3G and 4G networks are loaded down and very spotty. With the 3GS I could turn off 3G and switch to EDGE and go from 1-2 bars to 5 and go from 4-10 dropped calls a day to 0. With the iPhone 4S and 4G, I still have the spotty service and dropped calls, but no easy workaround.