My 35 f1.4 Nokton was coded by the previous owner with a rotary tool, so it looks a bit ragged but works fine. A Zeiss lens from PopFlash was sent by them directly to John Milich for milling and coding the original flange. About 10 other assorted lenses were sent to DAG for coding and a focus check/adjustment as needed.
I am happy with the work from DAG and feel comfortable that the lenses have been brought into closer agreement with the spec. That includes an older 35 Summicron, a 28 Summicron, a 21 Elmarit ASPH, and 35 Summilux ASPH, several Zeiss and CV lenses. Most needed some focus tweaking, and he did a CLA on a couple too. I think this is a good way to go about it.
I do like the lens ID (or a close facimile) to appear in the EXIF for my own convenience.
However, reading Sean Reid's reviews, it seems that many lenses are fine (or even better) on an M9 without coding, as the camera's corrections for corner color and vignetting are too strong.