I haven't used an ETR, but have an SQB (6x6 - which I presume works similarly), I also have a Mamiya 645 Pro (with prism & winder). If you shoot verticals on 645, you probably need a prism (metered or not), as trying to line up one with a WLF is an exercise in frustration (futility?).
The SQB does not have an instant return mirror, there's an LED in the finder which flashes when the shutter has closed (useful when on shutter speeds of several seconds), the mirror returns when you wind the film. Sounds simple enough, but it's something I had to get used to (especially if you miss the LED flash). I haven't used a meter prism or a Speedgrip - however I suspect the outfit would be somewhat clunky compared to the 645 Pro (with prism & winder) which feels just like a big 35mm SLR. The leaf shutters in the Bronica might be a help with flash sync.
The lenses are very good - all mainstream MF lenses were at the time. I'm not familiar with those for the ETR but the SQ series sometimes need an extension tube for portraits (closer focussing 'portrait' lenses were available but were very expensive).
The only problems I've encountered with the SQB has been a couple of used lenses - the leaf shutter was slow/sticky and didn't respond to the speed set on the body. You can check this by removing the film back, putting the multi-exposure lever on 'continuous' (this by-passes the body/film back interlock) and looking through the camera. Like many leaf shutter cameras, the lens mount has a cocking pin that must be in the right position before mounting the lens, the body has a similar pin which moves as the film advances. Oh, and you can't remove the film back if the dark slide isn't fully inserted - make sure you get one with the camera!
In use I prefer the 645 Pro - it just seems slicker and less clunky but my aim was to get 35mm like handling in a larger format.
John.