"Horses for courses" has always been my motto on this subject.
While racing games and beat-em-ups are great on the 360, both online and offline, I find FPSes to be dire. I hate the controls, I hate having to sit in front of my TV to play them, I hate having to compromise on graphics.
I prefer using the best tool for each individual job
Although for people on a budget or wanting to keep clutter to a minimum, I guess the 360 will be seen as a 1-stop solution.