I've had a couple of Rolleiflexes and one Yashica-Mat. First, I know it's hard to tell without actually using one, but be sure you really want a TLR. No matter how good they are, you are not looking through the lens. Some might say these became obsolete years ago for a reason. I never did any test charts or anything like that, just standard 8x10 enlargements. I could not tell much difference between the Rolleis (one with a Xenar, the other with a Tessar) and the Yashica-Mat (Yashinon lens) on that size of print. The Rolleiflexes (and the Rolleicords are virtually the same but without the crank) definitely feels more solid and less tinny than the Yashica. CLA's on these things can easily be way more expensive than the camera, and most places, you will have to mail it out to the repairman, so, be sure you won't need a CLA soon after you buy one. Personally, while they are very nice to look at, I would much prefer a used 6x6 SLR.

To be honest though, I'm not sure it's that much of a step up in photo quality from a modern 35mm SLR, while at the same, having some significant disadvantages in comparison.