I got the run down from a mate of mine yesterday who went to Dimensions festival, so pretty much the same thing.
They are really tight on party substances apparently, she seemed to suggest there was like one guy selling to the entire campsite, but I guess that's not the case outside of the festival? You can get hold of stuff, but not very readily, and it's resi rather than buds at the greener end of the party spectrum. I was thinking I'd maybe smuggle a few gees of mandy in with my luggage.. or maybe just wedge a whole heap of smack up my bum, depending on the mood.
Booze-wise it's all wine and beer on the campsite but you can head into town for stronger stuff, assuming you decant it into plastic bottles.
She suggested all in all it wasn't a big-deal, a few nights were messy and I suspect the combination of a fierce Croatian summer, crystal clear water and the baddest of beats will combine to create something pretty special no matter what happens.