I've had a Hassy outfit, loved it, then traded it in on some 5x12 field camera stuff when I realized I wasn't shooting much medium format anymore. Everything the folks here have said before about the pluses applied - lighter weight, amazing selection of accessories and lenses. Personally, I LIKED the square, and I also liked holding from the bottom - I found it made it easy to shoot handheld down to 1/15th of a second or so. Might have even pulled off the occasional 1/8th. I never found the back interlocks fussy, honestly.
I recently got an RB 67. The multi-step thing I have to do to advance the film on my RB I do find incredibly annoying so far. That's the biggest complaint I have, aside from the weight of the thing. I CAN hand-hold it, but there's no way I'd want to do that on a regular basis, unlike a Hasseblad, which I dragged all over Spain for two weeks (Madrid, Segovia, Toledo, Salamanca, back to Madrid). I think I had the Hassy on a tripod three times, all of them for night shots.
I also have a Rollei 2.8 E, which is a nice carry-around contrast to the RB. Size-wise it's in-between the Hassy and the RB. Although ergonomically it is quite similar to the Hassy (hold from the bottom), the focus knob being on the left side and the shutter button on the right makes it awkward to focus and trip the shutter with the supporting hand (left) as I was used to with the Hassy.
All of them have had their quirks - I've had film advance problems with both Hassy and RB backs, but in both instances, the backs that had problems were worn out but not advertised as such. The Rollei's meter is out of whack (pretty normal for a Rollei of that age) and probably not repairable. It also needs a CLA - the shutter gets gummy in cold weather. The biggest frustration I have with the Rollei is the close-focus or lack thereof. The normal lens on the hassy, which is optically identical, focuses much closer. The RB can do serious macro work with the 90 normal lens, focusing much closer than either of the other two.