I've used a variety of SLRs and RFs over the years. And I'm still not totally swayed one way or the other.
Yes, I enjoy the window - like, bright view of a good RF (notice I said "good", because some are better than others).
Yet for many of my pictures, composition is important, out to the edge of the frame; RFs simply can't deliver the compositional accuracy of an SLR's TTL viewing. I know when reviewing a roll of film from an SLR that the composition will be precise; not so with an RF; because not all RFs have parallax correction; it's more hit-or-miss - which can be fun, in not knowing exactly what to expect from the roll. Which is okay if it's photos just for my private amusement. But I wouldn't want to rely on it for things like weddings or portraits.
I've found it easier to determine subject in/out of focus with the two RFs I own (Retina and Zorki IV) over my SLR (Minolta X370 & 700). The center spot focus screens on the Minoltas simply don't snap in and out of focus like the parallax of an RF.
Focus accuracy between bodies and lenses I've heard can be more problematic with RFs. Like cams needing to be machined. Especially with FSU; at least, that's what the word on the street has been, here on RFF, for years. I simply don't hear the same kinds of problems with people buying older SLR bodies and lenses.
As for weight, my old Zorki IV is pretty damned heavy, compared to the Minolta X370. And the Retina isn't far behind. I think this idea of lightweight RFs is pretty bogus, unless you're comparing them with Nikon F1s or other early, heavy SLRs. Or the big auto-focus SLRs. Most standard amateur SLRs from the '70s and '80s are rather light. But I wouldn't want to put either of them in a jacket pocket, honestly; unless I had two of them, one in each pocket, as a counterweight.
I really want to like RFs better than SLRs, but I seem to get consistently better images from the SLRs. But that's just my experience. YMMV.
~Joe
EDIT: Diopter adjustment is lacking on both SLRs I own, whereas the Zorki IV has it. Important for those of us with not-so-young eyes. But this is not exclusively an RF/SLR issue, as these features vary from one brand/model to another.