I've only been playing it for 3 weeks, and I'm still on the 'will not play other games' learning curve.
Very good fun, a bit like TF2, but darker.
Try playing the campaign mode (players vs AI) on a lower difficulty setting to get used to it.
Then don't do what I did - join up with friends who've played it since it came out, and will not ONLY play on expert.
PS. each campaign is split into 5 or so levels, with a save point at the end of each. Some of the levels are realy big!