Two separate opinions about M+. First for
@OneArmedBandit:
I had a weird experience with a M+ tyre. I got 3 punctures in close succession, but not one after the other, but within a very short period of weeks. I can't remember if I identified the cause. Because of reasons that I will cover in part 2, after the 3rd puncture, I flung the tyre to the back of the shed, and replaced it with something less annoying. The new tyre didn't get a puncture for months, so whatever the problem was it wasn't the rim or the rim tape or the tube. Eventually the replacement tyre wore out, so in an emergency, I put the M+ back on, bought a folding tyre, and vowed to replace the M+ when I got the next puncture.
And that tyre lasted for ever. The puncture proof layer is green (or blue, I can't remember which) and the tyre was more green/blue than black when I next had a flat. And when I did, it turned out the flat was caused by a failed patch, and the tyre was still good. As most of the black rubber was gone, I still binned it.
Conclusion: I don't know what caused the 3 flats. It wasn't the wheel, and equally the tyre was solid too. So I guess it was a random unlucky cluster. For all the people that have 10,000 miles without a puncture, someone is going to get 3 in 150 miles (or whatever the number was).
Conclusion for
@OneArmedBandit: if someone else was going to be replacing my tubes, I'd probably stick with M+.