As a leica owner, I would say build quality is the main pro with a leica body. Barely changed since the first part of the 20th century they are nearly all metal, so they are heavy, but they also rarely break. They feel quality, the paint work is matt, but has a silky feel to it.
They do not have a high quality metering system (on the M6 its a white spot painted on the shutter curtain that reflects light on to the meter cell), anything over a 135mm lens and forget about it, macro and realistically forget it, sports is also a bit of a fail.
What they do amazingly well is carry leica lenses, which are probably the best 35mm lenses available. They also carry Zeiss lenses, and relatively cheap but still very good Voigtlander lenses.
On balance, as long as you don't shoot super fast, far away or very close things, on 35mm film, the leica is the best tool for the job.
The M9 is arguably not up to the standards of modern DSLRs in terms of ISO, buffer speed, processing power, and costs abotu £7k, but it is the only full frame digital range finder currently available.
They are nice to have, but functionally limiting and extremely unadvanced. However if you are willing to live with all that, they are a pleasure to use
