I've had an M6TTL for 2.5 years now. Rarely have I used my SLR's since then. It's a fantastic tool.
However, although Leica's are said to be very tough, my experience is that there are things that can go wrong. Right out of the box, the 1/1000 shutter speed of my new M6 was creating funny patterns on film. About 1 year later there was another problem with the ISO dial. All these problems were fixed under the 2 year warranty.
Just because of these experiences, I would buy a new M7 or MP because they come with a 5 year warranty. I know, the price is outrageous.
On the other hand, if I knew a good repairman who could fix Leica's for a reasonable price then I would be very happy to get a used Leica M.