I know three people personally (as opposed to on the web) who own M8s, including myself. None of us has any plans to sell our M8s at present because:
• There isn't much choice if you want a digital rangefinder (dRF), and the M8 is still the second-best dRF.
• The best dRF - the M9 - is #^%@&* expensive!
• The M9 is not a huge improvement over the M8, especially if you're happy with crop factor digital cameras (plenty of people are - the APS format is not going away). Two of us have decided that the M9 is not a worthwhile upgrade and are waiting to see what new features the M10 brings.
• The M8 takes photos that are still technically as good or better than most dSLRs, and not far behind the best. In short, the M8 will always take very good photos: why replace it unless you have particular needs (e.g. preference for full frame, higher resolution than 10 MP)?
• The M8 cost shed loads (I had to sell my motorbike to afford it!) - some of us who bought it new are unwilling to sell it at a huge loss. I've heard that some M9 owners are keeping their M8 as a second camera. That's what I'm going to do when I buy my next camera - as pointed out, the M8 will always take excellent photos. And it's useful to have two cameras.
• If the M8 breaks, it can be repaired. Leica has publicly stated that it will continue to service and repair the M8 for many years. Most repairs will probably be cheaper than buying a new camera capable of matching its image quality or features (e.g. manual controls). (I sold my Epson R-D1 partly because I worried that it might be impossible to fix in a few years.)
Now, extrapolating from myself and two friends is hardly statistically viable, but I have a suspicion that a fair few M8 owners think the same way...