This is the big difference between the XBOX 360 and any other gaming platform. And I am not talking about being able to play with your friends online, only.
I am talking about that unique ability to turn on your XBOX 360, see who of your friends is online and what they are playing, and playing with them, or at least start voice chatting with them while you watch a movie or play another game.
Some of the best times I've had in gaming is when I could hear the bellowing screams of anguish as I crush my friends in Halo 3, or the hollow accusations of the losers as I pass them by in my lesser car blasting down a stretch of road in the Nurnburgring. "YOU ARE CHEATING!!" They would say, knowing that cheating is not possible on XBOX Live.
Add to that the availability of MSN Messenger support and you have a the full experience.
I wish more of my friends would pick up and XBOX 360 and "Jump in".