Where to Start on The Ultimates?

I was wondering where good jumping on points for The Ultimates is. (And Ultimate X-Men and Ultimate Spidey considering it looks like they are part of a crossover event for their next issues?) (Also, are the stories any good?)

I realize in 2002 there was a 13-issue story-arc, and 13 more from 2005. But then there's the current arc which has 12 issues so far. Can I just jump right in, or would I be better off seeking the other 26 issues as trades? Would volume 3's latest 12 issues be necessary, or can I start with the next issue's story arc (presumably #13)?