Got mine 3 weeks ago, aside from my CM9 experience with my HP TouchPad...which I mostly use webOS still to this day....I don't have a whole lot of "Android experience" but this is what I've come up with: (also there's today...and 2 more days of $0.25 sale apps, which change daily...so be sure to check there for some el cheapos..that normally aren't so cheap).
SwiftKey3 keyboard
exPDF rReader PDF Annotater...( can't say I really NEED it...but it was cheap)
Juice Defender...I'm getting a few more hours out of my battery. (also paid for)
Light Flow....check out the freebie..I bought it I liked it so much (change your LED notifier colors/patterns/which apps activate etc) (also paid for)
Lapse It ...time lapse movie maker..also bought.
Voxer ...turn your phone into a walkie talkie..cross platform with Apple users
Tweet Lanes twitter client.
I haven't bothered with ROMs at all...I hacked the sweet zombie jesus out of my Pre3 and TouchPad...but I think I'm just going to leave my SGS3 alone for awhile. I don't have the need to do it or the time to read up on what I need to do to do it. Maybe one day I could get around to it...just not anytime soon