I remember we had a lan-party a few years back playing COD4 and we were all on our MacBooks and MBP! Nobody was allowed to use the mouse. But with dedication, keymapping, and creative hand positioning it's... Possible. You'd still get owned by people with crappy fiver mouse though.
Hell, I played Mass Effect entirely with the Magic Trackpad and also clocked in 26 hours in Skyrim that way. But finally I caved in and get a Logitech G502 and a Logitech F710. Money well spent, and your experience will be light years better.
I'd recommend a keyboard also as MBPs get hot while gaming and heat up your fingers nicely! Yes, crouching sucks, better use C or CMD. Anyone knows how to permanently change keys and shortcut in Windows permanently to OSX style?