I used to play to an incredibly high standard in Counter-Strike (i.e. consistently against top5 clans in the country and top foreign sides), this also stands true for Counter-Strike Source, for which I played for the top side in the country.
I was a massive fan of Quake 3 Arena, Unreal Tournament, Unreal Tournament 2003/2004 and every Battlefield game, including Desert Combat.
Now, this wasn't meant as bragging, but it give you an idea of my gaming pedigree on PC in a competative arena, and also an idea of how much time I put into using a mouse and keyboard.
But my point is, anyone who says they 'can't get used to it' is talking rubbish. Yes it's different, no doubt about that, but people who aren't getting used to it aren't trying hard enough. Games on consoles are modified for use with a pad, playing them with a mouse and keyboard would make it dull and boring.