Russell Brand and Jonah Hill were FUNNY as hell with their characters in "Forget Sarah Marshall". Why? In my humble opinion, because they were background characters and had to work real hard to make every moment count, so they raised the bar and made sure their characters wouldn't be forgotten.
Flashforward a few years later, these two now have their own movie... which isn't bad, but isn't really great either. Where Sarah Marshall was touching and funny, Get Him to the Greek is dirty and forces its welcome. A few fun scenes do exist here and there, but none of them make for the fact that the movie feels tired, overly clichéd and is riddled from weaknesses in a script that only exaggerates further and further. It's been one or two years since I first saw it and I still have to watch it a second time... and I'm in no rush...
Special features are quite sufficient, though. Deleted scenes, interviews, behind the scenes, improv moments, etc. The DVD features an entire collection of special features too, so don't discard it.
Rent it first, but if you just LOVE these characters, then you may be in for a good time. Be warned that exaggeration is the name of the game, not plot development.