A Pentax 645n has in the past 9 mos or so become my favorite walking around camera for scenics. The body weighs almost exactly what one of my Nikon F5 bodies weighs. I bought it specifically to use with the much underrated Pentax 35mm f/3.5 SMC-A lens, which is tack sharp at f/8-11. Field of view is about equivalent to a 21mm in 135 terms. It seems to have less distortion than most anything as equivalently wide. Bought both lens and camera for what an ultrawide prime alone in 135 would cost, far less than my Nikkor 17-35mm was, used.
The one thing I would most like to mention about my jump to 645, is that the apparent sharpness is much better than expected from such a wide optic. Velvia and Astia chromes can be astounding.
While they're also great for other uses, the biggest factor in the popularity of 645's is that they were used professionally for wedding portraiture with neg film of higher speeds. Thus back in the day 400 or 800 ISO speed film was used for mixed lighting and dimly lit church interiors, etc with results that held up in 11x14s and 16x20s whereas 135 on similarly fast film was beyond falling apart at the larger sizes. Today this sort of work is being done almost exclusively with digital due to the ISO and resolution improvements in 35mm sized sensors outperforming 120 film (not to mention the lower per-frame media cost) under such conditions. There's a glut of very fine 645 gear being unloaded and flooding the market the past couple of years at prices that are a mere ten to twenty cents to the dollar of what this pro gear cost new. If there's a caveat here, it's that some of this equipment has seen a lot of actuations, even though it might still look pretty good externally. So it helps to know the provenance of the gear you're buying. (Then again, the same would be true of RB67 gear, often very heavily used in studio environments)