I have the game for both pc and 360, I'm insane like that.
I can't play the 360 version well, their controller is chunky and my reflexes are poor with it. Especially shooting. stupid trigger.
And to answer your question Gabby: its just the satisfaction of having them all completed. You get nothing when you get all achievements.
But because I'm done them, I mess around on the altered servers, lots of fun.
And the new content is expected in April.
But it doesn't look that impressive. No new campaigns, just a timed survival add-on where you get the infinite horde control, and see how long you last. Not my thing. And they're recycling all the old maps. There's only one new one for survival: The Lighthouse.
I am looking forward to the map-creator though. It'll be sweet to create maps/campaigns