Getting themes and deadlines has always been good for me -- forces me to write, because I have a tendency to find other things to do when the page is blank. I agree with others that the opportunity to read others' work was also motivating.
I felt like I ran out of gas in the last week, or maybe just didn't find anything in the sub-theme. But the group as a whole fills in the gaps when any one writer hits a block, and that provided both ongoing motivation and also took individual pressure off.
See Rod's work as part of SUPERHEROES, July 17, 19, 24, 31 August 2, 7, 14, 16, 21 Thursdays & Saturdays at Exit Theatre.