Just to add some much needed balance to this debate...
I bought an S3 at the end of May, and had it for 3 weeks. It's a really good phone, and the OS is fine once you get used to it.
But I did feel compelled to sell it at the weekend at get an iPhone again, and I have to say that I'm pleased to be back.
The iPhone OS is still a quicker and smoother experience than Android ICS in my opinion. Despite what many are saying in this thread, I actually found that I've been able to text a lot quicker than I could on the S3. This, I admit, is probably partly due to my familiarity with iOS.
What I found with the S3, was that there was slight lag with opening some apps, and the screen refresh rate with screens containing widgets was noticable. You never get that with iOS, though widgets are a useful addition to have.
The iPhone screen is the thing I'm most pleased to have again though. Whilst the S3 has a good screen in low light conditions, it's really bad outdoors. To the point that it's of no use in sunny conditions. I'm sure S3 fans will try to convince themselves otherwise, but on maximum brightness, the S3 screen is too dim when used outdoors. In contrast, the iPhone screen is bright, sharp and perfectly usable in the sun. I really wanted to like the S3, but the screen was a deal breaker for me.
I was also in two minds about whether it was too big in the end. The screen's a nice usable size for web browsing, playing games and watching videos, but I did feel a bit self conscious 'getting it out' in public, especially for making calls. It does take up a sizeable space in your trouser/jean pocket too. Would I prefer the iPhone to have a slightly larger display ? Yes, a slightly larger display would be good.
Don't get me wrong, I'd definitely recommend as S3 as a great phone. But in conclusion, I prefer the size, bright and sharp screen, apps choice and smoother OS of the iphone.