First I would refer you to several long threads on bags for Leica Ms. One here just recently and several over at the Leica User Forum. Try a search on Leica Bag .
Points of view seem to center around two key decisions. How nice can the bag look and how big does it have to be.
SIZE....most M shooters prefer a Satchel Style that will hang close to the body. These are typically 12-14" wide,8-10" tall and 4-6" wide. The narrow width is possible because of the M body size . You can go smaller but then you lose some flexibility to carry more than the camera etc. One key consideration is would you ever want to carry a small laptop? A 14" bag will handle a MacBook Pro...this can be quite useful when traveling .
NICE LOOKING..in the eye of the beholder . You will get some that feel the worse looking the bag the safer the contents. Others want there prize possession to ride in style and will go with a custom leather/canvas etc.
The tops bags in order of discussion .
1. Billingham Hadley .....comes in 3 sizes and various combinations of leather,canvas and nylon. Available in tan,sage or black. These are beautiful bags. The trend is to go with the black leather/black canvas or nylon to show some restraint.
2. Artisan and Artist ....comes in a variety of satchels canvas,leather combinations . The ACAM series is mostly canvas. The GCAM series is leather and canvas. They are all black but sport red nylon padded interiors.
3.Domke .....Jack had a set up with the J-803 satchel and an F2 padded divider that is just about perfect. Photo is available in a post by him about "posting photos" . More room than you need for one body. The smaller version is the F series satchels. Nylon or canvas. This is a real working bag set up and the best value . The bag is about $125. This is what I use when I don t want to be noticed .
4. Fogg ...the High Street model...custom made in France . Most expensive and they look it. The Luigi case of bags. A little boxier in shape (wider) and with a certain structure to the partitions. Some would say you should be wearing a sport coat to use one.
5. Any other Satchel with Domke or Billingham inserts .
Most bag mfg have satchel style bags that could work. Think Tank makes great stuff but their center is on Pro DSLR requirements.
If you look at the websites of the top 4 and focus on the size ..you will see the slight differences. If you want room for a MacBook You need a Billingham Large (some Pros work as well with stretching). My new GCAM-7200 will hold the MacBook but not comfortably.
Few have been able to constrain their kit to one body and 2-3 lenses..so I like the J-803 as the first bag even though the macbook doesn t fit.