For the last seven years, I have used the Tamrac Velocity 6x sling bag. I have used it extensively, so much that I needed to replace it with a new bag. And what better bag to buy than exactly the same model? Here they are, the new to the left, and the old to the right:
The bag comes with two internal dividers, which can be placed wherever you want with velcro:
Normally, I only use one of these dividers, to make one big and one slightly smaller room:
That way, I can carry one camera with a lens mounted, and an additional 1-3 lenses, depending on their sizes. In this example, I have the Lumix GH3 with the Lumix X 35-100mm f/2.8 mounted, and the Lumix X 12-35mm f/2.8 as a spare (click to see my lens reviews):
It is also possible to use the front pocket to store some pancake lenses. The front pocket can be used to carry memory cards and batteries, and has a clever system to recall which one is full, and which one is used.
The bag is to be carried diagonally across the torso. When in the rear position, it is reasonably level, so you don't need to worry about stuff falling out, even if you don't close the top. The wide band helps keep the bag in place, so that it does not slip around while walking or even cycling:
The bag slides under the right hand to the front position, where you can access the contents. It is also quite level here:
After seven years of heavy use, one of the lower corners has become quite worn:
Also, the zipper doesn't always close perfectly around the second turn:
To prevent the zipper problem, I guess you can avoid closing the bag completely unless absolutely needed. The extra friction around the second turn appears to wear the zipper.
I think the Tamrac Velocity 6x sling bag is very good for mirrorless camera use, where it easily takes one camera with 2-5 lenses, depending on the size of them. The sling design means that the camera is always easy to get, and the bag is reasonably weatherproof. It has proven to be very durable for me, and I expect to continue to use this type of bag for another decade.