I've had iPhones since the original and when I sold my 4 a few days before the 4S came out, I bought the SGII to try as I was very intrigued by android and that phone specifically. I'm not a hardcore apple "fanboy", both me and my wife have iPhones and a couple Apple TV's but that's it. I must say, I was extremely impressed with the GSII. I really LOVED the bigger screen and feel for the phone, the alerts were impressive and I've said it since iPhone 1, but I really wish Apple would put icons in the top tray for email/txt/etc like android has. The camera was great, loved the level of customization and the multiple home screens with widets. In the end however, I did return it for a 4S.
My main reasons were
-my current investment in apps/music/movies, ease of syncronization with iTunes (although most of my music worked with android, my videos did not and it was more time consuming to sync it all)
-sharing apps with my wife, not having to buy angry birds twice, lol
-Biggest one probably was that my work doesn't support android yet for corp email and calendar (not until they support sd card encryption)
I could get by 1 and 2, but 3 was a big one for me. Overall, I'd probably go back, however I'm hopeful the iPhone 5 will be what I hoped this one would be.