I have picked up a nice little ETRSi set in the past few weeks. I am quite pleased with it, and bought it after a lot of consideration.
I have experimented with quite a range of cameras and formats in the last couple of years, looking for what was most productive for my style of shooting -- including digital, 35mm RF, 35mm SLR, medium format folder, TLR, 6x7/2"x3" press, and 4x5 press. I settled on medium format as possesing the greatest image quality possible in cameras that I actually wanted to use. I knew that I wanted some kind of ground glass viewing, for contemplative work, and the possibility of a prism for more action-packed situations. Finally, I also hope to have a leaf shutter, to offer a fast flash sync speed.
The process of elimination lead me to.... Hassleblad! Which wasn't really feasible (even with the "low" current prices.) Beyond the name, the other camera system that offered the same use-value was Bronica. I knew the 645 format wasn't a problem for me, because when I shoot 6x6 I will often crop for an 8x10 anyway. The ETR system, which was SO popular for working photographers, is now so common on the used market that the currect prices are great. I picked up a very complete ETRSi kit, with 75mm and 150mm lenses, for half the price of one Hasselblad lens.
The build quality on the camera and lenses are great. Definitely on a par with the best 35mm pro gear I've handled. The electronic shutters also gives me confidence that the camera will keep working without constant need for adjustment. As for image quality, I am very pleased. The lenses (even my older MC series) are a step up from the 4-element Yashinon in my Yashicamat. The waist-level finder works a treat. It is bright and accurate, with a lovely critical focusser. With the regular prism and speedgrip on the camera, it handles like a giant Spotmatic! The prism is large and bright -- much brighter than I have seen in the Pentax 645. I don't know anything about the metered prism, as I prefer to use a handheld meter anyway.
Given your 4x5 experience with the big ground glass, I think a Bronica (ETR or SQ) with waist level finder would be a very natural way of working for you.
Economically, also makes a lot of sense. I have my 120 processed and scanned at the same lab as my 35mm, and for the same price. The high-res minilab scans are the same resolution as a 6mp DSLR, and 645 represents the sweet spot for those scanners. The frame size matches the native scan proportions and provides the maximum film area for the scanner to work with. The result is lovely tonality in your "digital proofs." Of course, 120 is also very easy to process yourself. My home darkroom only does up to 6x6, so 645 works well with the equipment I have and will let me easily print "ideal format" as they say.
