Pros: Well-imaged sound, even response, easy to drive
Cons: Uncomfortable, ugly, very revealing
I listen to all types of music: from mixtapes of dubious quality from DatPiff to FLAC rips of half the MFSL library. I'm generally listening at my desk, though I also listen walking the dogs, walking myself, on the subway, lying in bed. About 65% of the music I listen to is rap, 10% electronic, 25% rock/pop of some sort. I'm a music critic, and my usual tools for evaluation are the Audioengine A5 speakers, Denon AHD2000, Shure SE215, Apple iPhone earbuds, and now these, the PSB M4U1. Honestly, I just listen to a lot of music with whatever's best at hand.
The PSB M4U1 are some of the best headphones I've personally heard, and compare favorably (to my recollection) to the HD650 and IE8.
I've found the headphones to be very precise and revealing. They don't necessarily present a wide soundstage, but the separation is very good. Different elements of a track, or different instruments, all receive pretty clear characterization. I've actually noticed a lot of different ambient sounds in studio recordings since using these.
The treble isn't at all rolled off, so they seem a little 'sharp', but I suspect that's because I'm used to slightly muddier or bass-heavy earbuds like the IE8 and SE215.
As for the bass - it's good. It's certainly not 'powerful' or overwhelming, but it seems accurately accounted for. When you've got the volume up, there's a somewhat physical bass 'hit' (sub-bass, I guess some people might call it?). I don't mind super-duper overwhelming bass; I turned the bass setting on my IE8 all the way up, and sometimes wanted more. I find the PSB M4U1 more than sufficient. They don't bowl you over, but the total sound package is such that the bass doesn't need to overwhelm to have a solid presence. As you can see from the length of these paragraphs, bass is pretty much the second- or third-most important thing for me, and these have it.
The PSB M4U1 have revealed some flaws in my music that I have previously missed. That's sort of a plus and a minus. Since I bought them to be an 'every day' sort of headphone, I'd like them to 'enhance' my experience. But if the source material (all FLAC, 320, or V0) is flawed, well, it's not their fault. Still, I didn't notice this sort of resolution with the HD650 or AHD2000. That could either be a plus or a minus.
Along the same lines, I think it could be that I wasn't (somehow) driving the HD650 sufficiently. I've had no such problems with the M4U1. They seem to drive easily, though, and I may sound like an idiot here, I haven't amped them. Maybe they sound even better amped. Compared to the un-amped AHD2000, they sound much better: clearer, better separation of sounds, more accurate bass.
Fit and Finish
OK, this is where the PSB M4U1 lose half a star. I do have a pretty big head. I wear something like a 7 3/4 hat. My ears might be a little big. Whatever. The tips of my ears hurt after wearing the headphones for more than an hour. Given how great they sound, this sucks. I never want to take them off, but I've found after a week that I have to take frequent breaks or suffer sore ears. The headphones also look pretty ugly overall, and they look stupid on my big head. (But hey, if you have a pinhead, maybe they'll look cool on you.)
I got the gray pair. They look like a really bad knock-off of the HD650: they're all plastic, no metal, and just a plain, upside down U shape like wearing a cross section of a bucket on top of your head.
EDIT: I just remembered another slight cavil. The cable with the mic/remote is very long. Since the cable is inherently for portable use, I wish it weren't like three feet long. Also, the mic/remote is really just a play/pause button; there's no volume adjustment on it. I still wouldn't knock the M4U1 down to 4 stars over this, but it's close. The sound is the most important thing, but obviously the headphones must be used as objects in the world, and I find their use to be highly problematic.
Given that these headphones hurt both my body and my pride to wear, the fact that I'd still rate them a 4.5 should give you an idea of how great they sound. They sound great. That's the greatest attribute for a headphone. I don't care if I look like a dork and have sore ears. It's worth it.
BUT - If you're on the fence from a sound perspective between these and, say, the Sennheiser Momentums or something, I'd say pick the other ones. I find these uncomfortable, ugly, and a chore to use. They do sound good, though.