I still haven't finished a single Far Cry game, but the open world, graphics and new game mechanics always have me interested. This latest iteration doesn't seem to disappoint either, the dream/past realm looks pretty goddamn sweet.
The Keys to Kyrat part seems really interesting, invite friends to play multiplayer even if they don't own the game. It's only for two hours, not sure if you can invite the same person back on a different key.. but it's pretty sweet either way. Download size would probably be a bitch, but if it's a sign of things to come in the future I like it.