Great thread, and I'm glad to see it hasn't devolved into an Android vs. iOS debate.
I've currently got the 4S and GSII and I have to say, I love them both. My 4S is jailbroken and my GSII is rooted and running a custom ROM, so I do like to tinker. I'll agree with others here that have said that iOS is still smoother...but the GSII is great too.
I do like the customization that you get with Android but there are things that have bugged me (although I've picked up some good hints from posts in this thread). One is email on the GSII. I can never seem to get it working exactly like it does on iOS (which is pretty flawless in my opinion). It comes close, but it's not as good (again, IMO).
I'll typically switch from the 4S to the GSII when I get bored of it, maybe once a month or so. But I do seem to always come back to the 4S for the simplicity of use. The killer, for me, with the GSII is the battery life. While it's not HORRIBLE, it's nowhere near as good as the 4S. I don't have a problem plugging it in while I'm in the car or at my desk at work, but it does annoy me that it seems to drain twice as fast as my 4S.
If Android can ever get the battery life figured out, I'll keep right on buying their devices, because I like the change of pace. But I honestly don't ever see myself not having an iPhone. Can it get boring? Yes. But it's just so well-done and easy to use that it's hard to let go of it for any length of time.