Finally got the GS3 yesterday! Loving it now, but yesterday was a lot of fussing with downloading apps, creating accounts and fiddling with settings.
I think I finally have it the way I want, and I think that with the automatic updates, I won't have to do much more than just use the phone! In that way I think it's better than iOS, where you have to check for updates to apps if, like me, you're slightly OCD about stuff like that.

Coming from iOS, I had a hard time grasping the concept of widgets, but now I love them. I wish some were better designed though. I downloaded Beautiful Widgets and replaced the standard Samsung weather/clock widget.

I think I'm OK with TouchWiz. I downloaded a few other launchers, I think Apex and also one more. But there wasn't a huge difference that I noticed, besides the home screen transition effects.

There's a few things that I kind of expected to work off the bat, but didn't. Hulu's app doesn't seem to be ready for the GS3 yet, at least in Japan. Gameloft's catalog of 3D games is severely lacking in the Japanese Google Play store (or for the GS3). Those things are irritating, because I'm going on a 4 day trip from tomorrow and I expected to have things like Hulu up and running.

S Voice and S Beam are missing from the Japan model. This kind of irritates me, that they would have some features for one market, and not others. Especially a flagship feature like S Voice. It's not a dealbreaker, and even I can predict that I wouldn't actually use it in daily life, but still.
:-! Loving the big screen, but still waiting on the Hulu app to be able to fully enjoy it. YouTube video quality seems better on Android, as well.

Loving the Google Maps and Google Navigation, especially on that badass screen. Yesterday the walking navigation helped me get to a job interview on time. And the GS3's GPS seemed to be faster and more accurate than the iPhone's. The maps are the single biggest reason I ditched the iPhone - their new iOS 6 maps look crappy to me.

S Planner seems OK but I preferred the iPhone's calendar app. Perhaps the stock Android calendar is better than S Planner but I haven't seen it yet.

Some of the apps are better on Android than iOS, and vice versa. For example, PayPal's iOS app was updated a few months ago and looks awesome, while Android still has the old version. But This American Life's app is better on Android because it lets you download podcasts directly to the Android music app.

The stigma that iOS apps are beautiful and Android apps are ugly isn't necessarily true. iOS apps do seem a tad more refined and their interfaces are more standardized, but Android isn't as bad as people have said.
:-$ Some of the GS3's motion gestures, like turning over the phone to silence music, annoyed the crap out of me. When listening to music in bed, I hold the phone face down. After 30 minutes of wondering why the music kept cutting out, I finally figured out that stupid gesture was to blame. So that's been shut off. Made for humans, my ass.
Overall though, I like the phone a lot. Glad I switched. Especially those maps! Gosh those are good.