I purchased the Rolleiflex SL-66 ten years ago, in September 1999. It is my most versatile camera, and I have learned more about composition with this camera than any other.
My first camera was a Sears Roebuck 126 instamatic, which I got when I was six, in 1971. I got a 110 pocket camera in 1979, but instinctually I compose in a square, so for me, even though I'm a professional photographer, using the SL-66 is like shooting with a sophisticated snapshot camera, and I feel more so than any other camera, it is an extension of my eye.
My likes:
The interchangeable film magazine backs. I own three, which are always loaded with slow b/w, fast b/w and a color film of some variety (either Velvia 50 or Vericolor III).
The Zeiss Planar lens. Sharp as a tack, and few moving parts.
Nice, heavy camera. This is important in wind, and when cradled in two hands, with myself as the tripod leaning against a wall or telephone pole or tree, I can go down to 1/8 second and get well-focused negatives in an emergency moment when I don't have a tripod.
Fast groundglass. I don't have to shroud the viewfinder so much.
Superb camera for copy stand: Makes excellent duplicate negatives from original prints, no loss in detail.
Repair: Harry Fleenor in Oceanside, California, is tops. Very professional and lasting repairs.
Drawbacks:
Sort of touchy: Not a rugged outdoorsy type of camera. A few rough bumps and it's off to the repair shop. Best to keep in padded camera bag until needed.
Difficult to find lenses, accessories, backup equipment.
Repair: Stay away from Marflex in New Jersey. Lousy repairs, my guess is they canibalize cameras for parts which are well-worn past their prime.