I have been "playing" on Health Month since mid-December, and I have to say - it's been fun. You create your own health rules and decide what you want to track, and then you check in every day or so to see if you've reached your goals. There's a daily email - if you want it - to remind you to "play". It's a good way for me to track progress, to be accountable, and to have some fun while doing it.
I think they're trying to make it social - a little like Facebook - but haven't really gotten it there yet. It's a hard balance - it's more fun to play with others, but sometimes you don't want to admit that you haven't eaten in your calorie goals for three days to all your friends. But it would definitely make it more fun.
I guess I like HM because you get to compete against yourself. That's my favorite type of game. Especially when it means I lose weight at the end!