Experiences will vary. Don't do the big resort thing: you'll hate it.
However, you might love what I did. I went to Negril (stay away from Montego Bay and all that shit) and got myself a room at a little place on the cliffs (stay away from the beach if you don't want hassles and cops). There was a lot of so-so weed, but I quickly met the right people and got hooked up with some nice bud. For 1000 Jamaican dollars (say, 16 US bucks or so), I'd get about 2-3 ounces of this really nice bud. I smoked like a fucking fiend, and went skin-diving all day. It was awesome.
DO NOT DO RESORTS!!! They defeat the whole purpose of Jamaica. Besides, all the horror stories about maids calling the cops on you happen there.
I stayed at
Heart Beat Jamaica which is on the low part of the cliffs, not that far from the beach. Friendly staff - even the gardener sells weed, but it's pretty harsh stuff, but he'll give you a lot for your buck if you're looking for quantity. The owner is a Canadian woman who'll gladly point you in the right direction if you're looking for weed. If you want nice bud, though, there is a bar next door called "On the Rocks". There is an old rasta who works there - hell, pretty much lives there. Ask him to introduce you to "Sy", and thank me later.
My best contact there unfortunately left Jamaica.