I updated my now old and unused 3GS to iOS 6 beta 1 just now.
I am actually finding it a bit smoother and zippier than iOS 5. I did an "update" in iTunes to iOS 6 so all my apps and settings are there. It doesn't stutter or hang as much.
While I may be on Airplane Mode without a SIM, I can say that the standby battery times have improved greatly, and usage time is improved as well. I can't attest to that with an active phone service on a 3GS though. I'd imagine there'd still be a bit of an improvement.