Pros: Overall sound quality, decent value
Cons: Some peakiness
I had a pair of M2s that were my beater IEMs for a while, but my years of abuse eventually took their toll and they came apart at the strain relief. I decided to try the M3s since I liked the M2 sound (smooth, warm) and heard they might be an improvement on the sound.
Starting from the bottom, there is a noticeable but not overly distracting mid-bass hump. It adds some nice punch to the bottom end of most songs. Extension is decent, rolloff starts at around 50hz. All in all, I would say the bass is a good balance of fun and articulate.
The mids have some slight peakiness in the 1.5-2khz region which I notice mostly when listening to classical instruments, specifically piano and flute. It gives the notes a sort of unnatural resonance, almost metallic sounding. Sibilance is not a problem I have with these phones. thankfully, so while their upper end is a little more pronounced than the M2, they're not harsh.
Treble is fairly neutral and extends decent as well. The separation and imaging on these IEMs is very good as well, probably the biggest difference from the M2, allowing the treble to breathe and providing a more balanced sound overall.
Isolation isn't that great overall because of the venting, but as is usually the case when music is playing it's hard to hear anything else.
The shape of the driver housing is meant to be worn cable up, I think, as wearing them cable down for an extended period of time can be pretty uncomfortable.
To sum it up, these are a very good option as far as sub-$100 IEMs go. Depending on what sound you're going for or your budget options, you may find the other two models to be better value. If you're looking for a smooth balanced sound these are excellent performers.