Lol, most games I play campaign first. Except for CoD4 :P I played the prologue and then jumped right into Online mode. Im on act 2 now on cod4, but I still barely played campaign.
Im not buying R6V2 because I think CoD4 will still be better. Unless R6V2 pays close attention to CoD4's online then out does it, I won't get it. Plus I like the aiming down the sights and Rainbow Six shoots at the hip all the time.
But you still haven't gotten The Orange Box, but you shold really play Half-Life 2, Ep. 1 and Ep. 2 instead of jumping straight online cuz its great.
There's also Bioshock if you want a great single player experience, though I haven't gotten it myself yet.