Former Bronica owner, current 6008, Mamiya C33, and GX680 owner. Obviously the latter is out of the question for your purposes!
Yeah, MF has exploded in price seemingly, though I got the Fuji for nearly nothing. I'm also a big fan of 6x6; 645 is too close to 35 for me, and 6x7 is a bit big in gear size, plus, I just like working with the square format.
The Rolleiflex is really quite nice. Motor drive, builtin average/spot/multi-spot metering and P/S/A/M modes. Definitely pared down controls for them, compared to a late-model 35mm or 645, but it's nice and simple to use. As was pointed out, no need for a metered finder. Huge plus there.
I rolled-my-own battery cell replacement and built a charger from scratch, and I've only had to charge it once since September.
Lenses are the same stellar Zeiss optics as the Hasselblad, but a bit harder to come by and seemingly a bit pricier. I lucked out finding a 50 and 150 for cheap (and insurance paid for them). Schneider lenses go for a prettier penny. The lenses for the older SLX are a bit cheaper, but you lose open-aperture metering.
It's a BIG camera for a 6x6. Almost as tall as my Mamiya, and heavier, and certainly noisy with the motor drive. The action grip is useful and I'll street-shoot without a strap easily.
It was a surprisingly wide range series with a lot of accessories that are pretty hard to come by, at least in the US—shutter controllers for using lenses on a technical camera, digital backs, interval timers, etc.
It is sort-of officially supported in the US still under Rolleiflex USA. If you're seriously looking at it, take a look
here for a model comparison, or shoot Eric Hiss at Rolleiflex an email, or feel free to ask me, after I did an embarrassing amount of research on it. There's an impressive number of models in the 6000 series, from the older bare-bones 6002 to the 6008AF.
The whole setup was a replacement for a Bronica SQ that was stolen this summer; prices for that series have jumped a bit as well, but I'd still recommend it too. Very light for what it is, and felt solidly built—its no RB67 but no toy either. Honestly I felt like it was lighter than my old Nikon setup. For some purposes, I certainly miss the non-instant-return mirror and winding the film by hand.
Some old published reviews felt their later PS series lenses were sharper and a little higher contrast than the Zeiss contemporaries. Metering and aperture-priority auto with a number of different prisms.
But as for the Mamiya 6: if you like RFs, get it. That's more of an SLR-versus-RF argument hashed over many times, but as I said, if you want an SLR for things like macro, or TTL flash purposes, nominally the Rolleiflex offers that but you'd be hard pressed finding the accessories.
I don't see many of the 6s out there—a lot more of the 7s around, but I feel like the 6 was a better camera in many aspects. Right after I built out my Rollei kit, a full Mamiya 6 kit showed up at a local shop and I can't say I didn't have a few regrets.
For me, I prefer RFs in general, but I shoot a lot of architectural work on MF and a ground glass took precedence.