...say I! I've just spent the last two hours co-editing a joint paper with my super-fabulous professor. And it was a good editing session - complete with giggling, wry remarks, and references to Wile E. Coyote.
There really are days that I love collaborating, I swear it.
Also, bonus points that this is effectively a chapter in my thesis.
Edit: And then I look at the other thesis chapter my advisor gave back to me and get very, very tired. Sigh.