I have my Galaxy S2 along with my new iPhone 4S.
Things i liked
-Huge screen, super clean, very vibrant
-So, so so so fast! Everything you do on the phone is quick. No slowdown at all
-Very, very good camera (front and back!)
-Android OS is great
Things i didnt like
-Typing on any keyboard that comes with the device is annoying as it doesnt detect some typing due to an overly sensitive touch screen.
-Android OS is stuck at 2.3 and although the TouchWiz 4.0 skin isnt bad, its not great either. Simple applications like MMS/SMS are terrible and look like crap. Button placements for "Send" are all wrong and it forced me to have to rethink the way i used the phone to send texts'.
-Sometimes odd battery behavior. Sure the battery was great, at most times. But there was some instances where the phone would reboot itself at 60-55% left,only to restart with 10% charge left. Very odd
-Android Apps. This is a multiple part issue, but some apps work, and some dont. Some look ok at best, some look terrible. Some are paid that work as advertised, some are paid that dont work at all (yes i know you can refund). Most look like they have been coded using Windows 3.1 as a design platform (as someone here put it).
All in all, this is a fantastic phone. Does everything you need it to do. Until CM9 (Cyanogenmod 9) releases, it will be in the box waiting for it. For me personally, nothing out there with the exception of Windows Phone has the polish and cleanliness as does iOS and the iPhone. I think that once Nokia releases a real phone (not the Lumia 710), it will be a great competition. Those are my thoughts.
TL;DR - Android OS is buggy and sloppy, GS2 is quick but has its flaws; iOS is complete and clean as is Windows Phone.