I’m not a fan of Omnifocus, or many of the paid apps out there. Not because they’re not good apps, but because, in my experience, there are no GTD apps that will do exactly what you personally need them to do. There will always be tweaking involved, and I’m not made of the money needed to test out all of the paid options. I’ve done some trials and never been particularly moved to jump in. That said, I know many people who swear by them, at least for a while.
I love Dropbox — but I use it for file storage and mobile access, not as a collection tool.
iCloud doesn’t do a whole lot for me, at this point. I like the ability to download apps to my different iOS devices, and I think some of the other saving/syncing features are good, but I don’t like the pricing tiers, and use Gmail for contact, calendar, and email syncing.