I have seen successful M6 patients posting on the internet, but don't know any personally. I work with spine patients professionally and have had 2 clients with failed M6 configurations that required removal of the discs. Please note, that I have a SELF SELECTING negative population for some groups of clients, so my experience is not a reasonable cross-section of the population, so take everything I say with a grain of salt. I am not a doctor and don't have a medical background except that of a part-time spine patient from 1997 until 2000 and a full-time spine patient since then.
I gained a lot of info on the M6 when it first appeared at the Spine Arthroplasty Society's annual meeting 2 years ago in Berlin. See about my breakfast with one of Spinal Kinetics' EVP's after the conference.
SAS Berlin blog
I think that the most important aspect of M6 for multi-level consideration is that it's a mobile-core device. I like mobile core devices for single-level procedures in configurations that don't have suspect adjacent levels and don't have substantial instability. From my experience, IMHO, mobile core devices are less desirable in multi-level configurations because of the possibility for one core to get forced one direction and the next core to get forced in the opposite direction, creating a substantial spinal balance problem. Please take everything I say with a grain of salt... I'm not a doc... not an engineer. I just share my experience. I am not suggesting that they don't work in this configuration. I'm just suggesting that if there are complications, the failure-mode may be aggravated by the mobile core.
Unfortunately, I'm about to undergo a 4-level cervical (and thoracic at t1-2). I wish I was not back to getting the patient's side of this industry, but I'm going to be one of the top ADR users, with my 2 Charite's in my lumbar spine, presumably 4 ProDiscs in my upper back/neck and my wife's 2 lumbar ProDiscs! Note that with everything I said, I have a very successful multi-level, mobile core configuration in my lumbar spine. All paths come with possible success and possible failure. Do your homework... make informed decisions.
All the best,
Mark