I have an M40 and I love it. It's incredibly robust in appearance and I imagine it is just as robust in reality.
When I first received it, it would fail to go completely into battery on occasion and that prevented me from carrying it. However, I recently replaced the captured spring with a solid spring guide with new spring and it's been 100% since.
Considering the malfunctions completely disappeared after replacing the recoil rod/spring, I am still under the impression that the captured spring denied enough tension towards the end of the slide's travel to ensure the slide went into battery.
This failure to return to battery issue is not the norm with M-series standards, so don't let this scare you off.
I think the manual safety is quite clever and it works wonderfully on mine. My only gripes with it is that it required two hands to engage it and it requires a trigger-finger inside the trigger guard to disengage. For many, this poses no problems. However, I keep the finger indexed outside the trigger until I'm ready to send lead downrange.
Of course, there's no rule saying this unique safety must be engaged, so it could very well be carried much like a Glock with safety off (with the primary "safety" being between the ears).
I don't know what fair prices are for these M series today, but I very much enjoy mine.