The M5 was "hated" by Leica purists because it didn't look like a traditional M camera and because it was "too big". Its sales were low in part because of that and because Leica had the CL on the market at the same time. The CL was smaller, had the same spot meter and continuously variable shutter speed on a similar dial setup, was built to a lower price point, and stole a very large part of the M5 market. The CL even has the same one-side strap lug set up. Low sales and production of the M5 make it rare, so the collectors value is rising relative to other M's. People are also finally realizing it's an ergonomically brilliant shooter's camera even if it's different from other M's. When the M4-P came out, Leica had dropped both the CL and M5, so it wasn't competing with a cheaper body and it "looked like a Leica", so sales rose again.
Originally Posted by 2F/2F