I've never used the Bronica RF645 but on the strength of my ETRS I would wager it's a good camera. However, the lens selection is purposefully limited and if you don't plan on buying more than one lens, you may consider just getting the GA645 in the focal length of your choosing (45mm, 60mm or 55-90mm zoom) and save space.
The meter of the GA645 tends to get fooled by bright skies but is otherwise accurate in my experience. The autofocus motor makes a very audible grating noise and the film advance is a loud, high pitched whine. Not for use in quiet spaces but if you're outdoors it isn't as loud to bystanders as you might think, being right next to your head. You can mitigate the focus noise by zone focusing. You can also make the camera wait to advance the film by keeping the shutter button held down after you take the shot as you scurry out of earshot. A GA645i costs more money upfront but is supposedly a bit quieter and gives you 16 shots per roll instead of 15.
One thing to be aware of is that, for people like me, the increased size, weight and complexity of medium format cameras slow me down. As does having to meter and adjust exposure manually. As does only having 15 shots and knowing I have to pay for processing. I am more deliberate shooting medium format than 135 or digital (unless I'm traveling out of town I purposefully use an SD card with only 2GB to discourage wasteful shooting). The GA645 by default is fully automatic and the ease of use can make you cavalier like you're shooting a 135 point and shoot. Knowing I only have 15 shots has largely helped me focus on shots worth taking.
Here are some of my favorite shots taken with it so far. Developed in TMAX and scanned with a Noritsu by Indie Film Lab.
Ilford FP4+ rated at 50 ASA.
Untitled by
Pierre Saget, on Flickr
Expired Tri-X rated at 100 ASA with EV+1 compensation.
Untitled by
Pierre Saget, on Flickr
Expired Tri-X rated at 100 ASA with either EV+1/2 or +1 compensation.
Untitled by
Pierre Saget, on Flickr