I'm mostly bored with Minecraft, but play occasionally on a polite cooperative server with limited resources and very creative people. Our regular server was down, though, so we went to a temporary new server with no rules of politeness and an explicit invitation to PvP. It was fun, but not much fun, here's my observations for our group:
I've had the most Minecraft fun in weeks on the temporary PvP server today. I also had the most unpleasantness. It reminded me a lot of Eve Online, only without any purpose. Like M, I think PvP Minecraft is not for me.
Highlights: finding a skeleton spawner, building a drowning trap to collect lots of arrows. Dropping lava on various high points to watch it go. Lowlights: feeling bad for killing people. Losing all my stuff to S. Having my drowning trap blown up with TNT by R. Feeling bad for trying to lava bomb R's house only to find it was someone innocent. (Sorry, F!). Then the highlight of them all, the best reward: killing R, looting a stack of 64 TNT, then using the TNT to blow up all the diamond armor he'd dropped just as he was coming to pick it up. Take that!
PvP Minecraft was sort of fun, but the moment you get too engaged or take it seriously it's upsetting. Eve Online is like that too and I came to enjoy it, because the tension of "this is awfully real and someone can kill me" makes for a powerful experience. I think the reason Minecraft fails as PvP is there's just no structure. No purpose to killing each other, no teams, no territory. Just the miserable pleasure of ruining someone else's day. I haven't pulled the legs off a spider since I was about 11, it stopped being fun.