I can't comment on the S series AF as I have never owned one; however, I can comment on Leica's AF attempts in other cameras, notably the XV, T and Q having owned or owning all of them at one time or another. I found the XV and the T to suffer from two AF issues, slow speed and sufficiently frequent AF error to rate those as unacceptable. By AF error, I mean that the camera indicated it had focus lock but the resulting image was out of focus. Because I often shoot with review turned off and do not chimp at the time I am shooting, it was irritating and disappointing to later discovered the image was lost due to mis-focus. The Q is the first Leica with AF that I have owned that gets AF right. It is fast in virtually most conditions including pretty low light and it rarely misses. It's AF matches or exceeds the very fast and accurate AF of the M43 cameras such as the Olympus OM-D EM-5II or other equivalent Panasonic models. The prior implementations were too slow for moving subjects and prone to a strange bug where the camera would indicate focus but nothing was in focus. A dive into the EXIF data would show the camera selected a far distance focus or an extreme closeup distance to place the AF and there was nothing in either area that was high contrast to fool the AF system.