We've been using the Photoflex boxes. I have another that I got with my Alien Bees, it's big but not as easy to assemble and doesn't come with extra inserts (with the exception of a second diffuser.)
Over all the Photoflex boxes are amazing to use. Quick to assemble, removable front panel, reversible liners so that you can go from just plain white, to silver or gold linings. Some of them come with circle inserts, they're sturdy, yada yada yada. I'm sold on them and will gladly spend the extra cash. Yup, they aren't the cheapest units on the market, but they're worth it.
We've got a 20"x30" and a 30"x40" Once we have the space it's a couple of 60" octogons! (Again, my wish list is getting longer and more expensive. [img]/ubbthreads/images/graemlins/confused.gif[/img])