You get:
Big Boss's sneaking suit from Ground Zeroes.
Bonus recruits for Mother Base, including any POWs you rescued during the course of the game.
As well as a possibility to revisit Camp Omega for a new mission in The Phantom Pain.
Nothing that is game breaking or gives you any clear advantage. Will pick up Ground Zero's again when I pick up TPP.
Pretty keen for the PS3 version. Would normally go for xbox360 but I sense multiple disks which really annoys me.
Here's a cool story:
I bought a Gigabyte HD5450 gpu for my wife's PC so that she can use two screens to work on. It cost R480. Takealot was kind enough to slip a MGSV:Phantom Pain steam code in with the card. I'm pretty sure that's a promotion for NVidia high-end cards, but I'll take the freebie. I've never played a MGS game and I've heard this one's awesome so maybe it's a good place to start.