Just to add my $0.02: I own both a Kiev 6C (an earlier variant on the 60) and a Mamiya C220. The Mamiya is definitely the better camera in terms of build quality, but the Kiev has its advantages. The main one, I think, is that the lenses are much less expensive and they cover a wider range (longer and shorter focal lengths). The Mamiya feels much more solid and is more reliable. Both were eBay purchases, but my Kiev arrived with frame spacing and shutter speed problems. I managed to fix these problems, but the film advance broke again after a few rolls. I've got to open it up and fix it again soon.
For more information on the Mamiya line, check
Dead Link Removed or
Dead Link Removed (I'm sure there are others, but these two seem to be pretty informative.) There's also a nice introductory piece in the April 22, 2006 "Amateur Photographer." (I've got a photocopy, but a quick Web search didn't turn up anything readily Web-accessible. Maybe a trip to your local library....) I'm sure there are lots more Web sites and information sources; I just happen to have these bookmarked or on hand.
This fotoralf.de page covers the Kiev 60. There's also a
very nice site devoted to the Pentacon Six system, which includes some coverage of the Kiev line, especially the lenses (which are interchangeable with the P6). The
http://www.kievaholic.com site is quite valuable, and especially its
"Kalibration" page, which describes how to do some common repairs. Again, there are lots more Web pages out there; I happen to have these bookmarked.
As a side note, the Pentacon Six might be worth investigating if you decide you'd prefer an SLR to a TLR. It takes the same lenses as the Kiev 6C/60 and is reportedly more mechanically reliable, although still not up to the standards of a Mamiya. It also tends to sell for a bit more on eBay.
As others have said, the Kiev is more likely to be mechanically trouble-prone, but if you get one that's in working order, it'll take fine photos. So will a Mamiya in working order. I've got photos taken with both cameras hung on my walls, and I'm satisfied with those photos. If I could keep only one of these cameras, I'd probably keep the Mamiya, although it'd be a tough choice, simply because I like very wide lenses. Although I don't yet own such a lens for my Kiev, I'd like to get one eventually.