I have currently an etr-s and an sq-am. The etr-s I've had for ages, and still use it for portrait work all the time. Especially when I need a high flash sync speed (it's a leaf shutter). The sq-am I've only had about six months, and only use it a little bit. The prism viewfinder is considerably dimmer than the etr, even with an Intenscreen aftermarket screen. But I do like square format and the motor drive is handy.
I've also owned an S and an EC. Both were fun but both died one after a few months of use. Brainiacs are good cameras, and dirt cheap compared to other MF cameras. I also shoot Manila 645af for wedding work. But I still bring out the etr when I need the sync speed.