I've recently re-entered iPhone land as well. My 3GS holds its own against other platforms pretty well. The only reason for this in my eyes is because I've been finding more/newer jailbreak capabilities, so I can get it closer to the way I really want it to behave.
I've tried other systems that I feel have better systems: Android (in some ways better, some ways worse), Palm Pre 2 (system is much better, but few apps to use and weird platform decisions from HP), BB has better physical hardware IMO (not internals lol); but the iPhone is always ahead for one reason or another. If you combine the lists of features, major developer engagement, huge list of possible tweaks and the system reliability factor together, you'll know it's worth at least keeping as a back up. If you want, get a Skype subscription or microcell. Google Voice aint a bad idea either. Sometimes AT&T will provide a discounted or free microcell if you tell them you get no service and ask nicely. Worth a shot lol. Even when I'm trying out another platform (and I do really like WP7 and webOS), I keep my old trusty 3GS on back up. I had a bad experience with the Droid 2, so I would stay away from Android myself in the future. I'd say that if you're really looking for a similar platform to switch to, try WP7. It is a simple enough system with enough features and apps and games to get you by. It's more customer friendly than Android, yet still has more hardware options than iOS. Focus or HD7S only, other AT&T WP are crap lol. Maybe the LG one if you like smaller/qwerty phones.