I actually just held it about 30 minutes ago. It feels so light. I would put an otterbox on it and a zagg invisible shield.
Maybe its the effect that apple has over me. I have a macbook, imac, ipad, appletv and iphone 4 and they all work so well together. I dont know.
Speaking a hardcore Apple fan that owns 2 Macs, Apple TV, every generation of iPhone, and iPad. I will say that Android is NOTHING like iOS. Is that a bad thing? Not necessarily. Android is far more customizable, and that's good and it's bad. In areas where the device should "just work" it sort of falls short, like the keyboard. Say what you want, but none of the keyboards outside of the stock keyboard(Jellybean) is even close to the level of the iPhone.
Secondarily, for a phone with a 1.5ghz dual core processor, and 2 gigs of ram, there sure is a fair amount of lag. Also having apps time our or randomly freeze is also no bueno.
Keep in mind, there is a slight learning curve. However if you are tech savvy, you should pick it up in no time.
With that being said, the 4.8" Super Amoled HD display is awesome, but strangely not as bright as the iPhones display. The phone while not a 1 handed device, fits nicely in the hand, and is super light. The screen scratches easily, but that's nothing a screen cover won't fix. I think my biggest gripe here is touchwiz. However keep in mind some of the Launchers do an excellent job of hiding it. While customization, and choice is good, certain things like the keyboard(which is essential to interact with the phone) should not be removed. The Samsung variant is absolutely horrible. I wish they at least gave an option to use the stock ICS keyboard. Overall the experience here is positive, but keep in mind the SIII doesn't have Apples level of polish. If you are really on the fence, and deep down don't want to leave the iPhone. I would at least wait for the new iPhone, and then make your decision. I have had no regrets switching over. I've customized my phone the way I like it, and now everything works like a charm.
Sorry for the rant guys, I just thought I'd chime in since I was in the same boat not to long ago.