Playing on-line with friends is win.
I love meeting and becoming friends with random people on-line though too (mainly RDR it seems) who I then play games with at later dates, I dislike having random people in my PSN or live list whom I don't talk to or play with.
ETA: I should add I'm not one for split screening with a buddy anymore, back in the PSone/2 days I was, but I hate sharing my PS3 lol. I don't mind on the Wii though, where you have 2+ players but you're not split screening.