I have(and use) 645, 6x6, 6x7, and 4x5. In the past I've been anti-645, but the nifty little Pentax 645 I picked up before Christmas has sort of changed that with how convenient and small it is. I still consider the Rolleiflex one of the most perfect camera designs ever made, and pull out my RB67 when I don't mind the weight. My SQ-A is probably most used on the whole, though. I'm going to shoot a plate or two this afternoon if I feel like braving the cold. Still, though, these small SLRs intrigue me.