When I pledged I went through what would be called structured informal rush. We had Meet the Greeks night, theme party, and pref party. The only time you had to see all the groups was MTG - after that, if you didn't want to go to a group for theme you didn't have to. After theme, you had to be asked back to pref. We still had bid day, quota, etc. Really the only diff between it and formal was that theme round. In formal you had to go to all of them. Oh, and during informal the sororities could have things at their (off campus) houses if they wanted to - during formal everyone's parties were in classrooms to make it more "even" (not all the sororities had houses, and the ones that did, varied in aesthetics).
I much prefer this to a week of COB free-for-all with no schedule coordinated by Panhel. Especially when first semester freshmen are rushing....grr.