I'd say there is a very large learning curve but the ranking system works nicely so it isn't a steep curve. I suggest starting off playing Space Marines for a few games as they're the easiest to handle if you're completely inexperienced... they're also completely badass. FOR THE EMPEROR!!
I haven't played competitively since before the big re-balance patch (the one that put more of an emphasis on t1) so much of that curve is in front of me again.
EDIT: Oh, and playing the campaign will not prepare you for multiplayer in any way.

