Well, I just received my M4U 2's. I have a pair of Sony MDR-1RBT, which are absolutely wonderful if you want a pair of headphones to wear around the house without being tethered by a cord, and a pair of Bose QC15's that I use for travel (but which are only mediocre as a regular headphone). I was intrigued by the strong reviews these headphones received, and the possibility that they could be a do-it-all headphone while traveling (sound great on the plane with NC on, as well as just listening to music in a hotel room).
My initial impression is that Bose has it nailed when it comes to a convenient travel headphone: light, compact and comfortable, awesome NC and decent-enough sound. The M4U's are pretty much at the outer limit of weight and size, although I suspect that the "pill" case does not have much more volume than the Bose case, so maybe it's just the weight (which is considerable compared to the Bose). Sound is a no-brainer, at least for regular listening. The QC15's always sounded anemic when just listening at home, although oddly enough, my impression on a plane has always been that they sound quite good, so maybe the SQ is optimized for environments like that. With the M4U's, there is a big difference (running it straight from an iPhone) between the passive and active/no-NC mode. Sound signature may be pretty similar, but a lot sharper and more dynamic. (I can't tell whether that gives a false sense of clarity or not; have to listen more.) I'll need to compare these with my MDR-1RBTs; both have on-board amps, although the Sony's obviously are dealing with a Bluetooth signal.
I don't plan on using this with a separate headphone amp, and have no interest in doing so. I've been considering going with a full-blown home rig and a really high-end set of headphones (like the Beyer T1s), but the likelihood that I'd use that set-up much given my lifestyle isn't that high, so if I could get reasonably close to that with the M4U's, I'll keep these.
Ideally, I'll like these better than the MDR-1RBT's, as that would give me two reasons to keep them: home listening when I can live with a cord, and travel. If it's a push, then I'd be more on the fence, as the QC15's are tremendous travel partners (and I can just carry a good IEM for hotel room use), and these are just plain heavy. To the extent I've been able to try the ANC, it does seem to work (you do get that suction-feeling), but also does degrade the SQ. We'll see.
Three things off the bat I'd love to see changed: (i) a good, built-in battery with USB charging (changing the AAAs is going to get old, and the battery compartment cover is not going to last); (ii) a cable that also controls volume for use with iDevices; and (iii) a less-Beats-like design, as the shiny black plastic has got to go.
I recently saw the Monster Inspirations at the local Tumi store. I really liked their design and compactness, but it does seem that their performance doesn't come close to these.