This is my sons thing and it really turns out cool I think, He loves doing it on the last day of a con (we always go 3)
I keep a decent stock of blanks for both commissions through the mail and at cons signings ect so I give him one and he gets everyone he can to sign it.
This one is from Albuquerque Comic Expo 2011
It has tons that I cant even remember but some are Brian Pulido, Artur Suydam, Matt Campbell, Shannon Denton, Andy Kuhn, and many others
This is his other one from Albuquerque Comic Con 2012
It has Brian Pulido, Jon Hughes, Nei Ruffino, Eric Basaldua (Ebas), Jamie Tyndall, David Harrigan, Larry Welz, Andy Carreon, David Mack, Patric Finch.
He likes it and I think its fun for him to do, He also has a sketch book he gets sketched in but that takes time and his little 6 year old patients don't last so long.