I was talking to Gordon Joly and Philafrenzy about this at the Wikimeet and I think the collective feeling was that contact before and after an editing session is important as well as the event itself.

I can certainly think of events where lots of people have turned up, but none of the new wannabe Wikipedians has gone on to contribute after the event - perhaps this could be improved by some friendly emails afterwards offering to continue the help offered on the day.

I think we're definitely learning more about how we do these things, and hopefully these events - whatever form they end up taking - will continue that. 

Chris