It can be legit, though not exactly easy. The cruise I'm going on next month is the result of one of these offers. It's a long story about how that happened - I'm not one who enjoys the hard sell and am pretty skeptical about what you actually get.
Here's what I can tell you, in a nutshell:
As mentioned in earlier posts, you'll have to withstand (or give in to) the hard sell you're volunteering to hear. Add to that: the "incentives" company that will be arranging the trip will set up an obstacle course of paperwork designed to make you voluntarily give up on it. If you don't give up, and keep playing the game (and perhaps write a diplomatically-worded letter or two), you'll get the trip.
It won't be free - taxes, "courier fees", etc. are involved - but it'll be cheaper.