well.. seeing that I'm one of thier main dealer/distributors I can tell you that the pickups are fantastic. A little bit on the expensive side though. The M series is actually one of the best selling models for me and I also do Bare Knuckle and Dimarzio.
Over all the lundgren M series is kinda like a Tone Zone with more output and more upper mids. It's got that "WALL" of sound that hits you like a battering ram in the chest no matter what tuning you have it in. It's a pickup if like your tone to be big, warm and aggressive, but like the Tone Zone, it can also sound woofy. If you generally prefer something with more top end bite take a look at the Bare Knuckle Nailbomb or Painkiller.
Here's a video of the M6 in action (the video is actually the trailer for Rob Rock's live DVD. CJ Grimmark, the guy playing the Caparison in the video is a good friend of mine and has been using the M6 for years.)
http://www.youtube.com/watch?v=Ey7TrjsBvG8