I have recently been using a pentax 645 as my travel camera. A 50 and 150 lens make a great combo that is no bigger really than a dlsr or big 35. Yes, the negative is smaller than some, but the lenses are great, the camera is well built, with a very good viewfinder, an accurate meter, and some automation.It makes for a great portable travel rig.
I have an RB67 and I love it. Yes it's heavy. No it's not too heavy for me to take it and a couple of lenses and a tripod on an 8 mile hike. Honestly I really don't understand it when people say it's too heavy to carry around. Granted, there are times when I want to travel lighter or my RB setup isn't practical for the situation and thats when I take my M645 Super.
As has been mentioned, you shouldn't try to solve everything with one camera.
I've been doing the same with my RB system - many miles, over mountains, etc. - for a number of years but now I need something weighing less because my age is forcing me to travel lighter, especially with overnight camping gear included in my pack. I turned 60 last week and discovered, while hiking a 2500ft vertical rise with my 3 son's, that 30+ lbs on my back is too much for me.
I've been looking at the M645 Super with 2 lenses, @ only about 5-6 lbs total (less tripod) - I figured that the weight is about the same as my Galvin 2x3 w/roll film holder and 2 lenses, lupe, lens boards, etc. - and gives me the option of shooting hand held rather than missing the shot due to a lack of speed (setting up).
"Pictures are not incidental frills to a text; they are essences of our distinctive way of knowing." Stephen J. Gould
What about the Bronica GS-1? Does anyone have experience with this camera system and can compare it to the SQA, RB, 500 C/M? iI happen to like square images but I can crop a 6x7 to make a square but if I crop a 6x6 I may as wll shoot 645.