It's quite fun, just don't burn yourself out too quickly on it. Also make sure you've got some friends to play with, otherwise you're really missing out.
seriously, buy it only if you feel super hype about the game, it's dead right now after finishing the story and doing the raid two times or so it gets repetitive and boring
If you play other games and do other things(life) as well and not just whore destiny all day you can get months out of it as is, but as said they are changing things trying to make everyone happy