Sequence diagrams are quite useful, static diagrams, not so much. I only find myself drawing them if I'm trying to do a necropsy to figure out hosed up object relationships.

The other useful diagram tfa doesn't cover is the finite state machine diagram. Handy, plus, they look cool.

As an aside, I find it's much, much easier to do with pen & paper than in any tool i've ever used (looking at you omnigraffle & visio!)