Went to Cayo Coco 4 years ago with the Mrs, I absolutely loved it -she prefers just lying by the pool for 2 weeks.
Obviosuly the downside is that you're confined to your complex at night, so I'd find yourself a nice hotel with decent entertainment on in the evenings. The part I enjoyed the most was seeing the culture on the mainland, absolutely worlds apart from what we'd know. Murals of Guevara and Catsro everywhere, searing heat and swarms of mosquitos all add to the experience if you fancy that!
The guys are right, there are a lot of Canadians there which I thought made a nice change from Germans and Russians that you see in Spain and Greece. We met a few chaps from Toronto and these guys HATE French Canadians with a passion, so stick with the 'English' Canadians and you'll be fine.