I just received mine today. I wrote a short post on Reddit (in r/headphones) with my first impressions. Below is a copy of my post from there:
First off, I apologize for the poor quality pictures.
Disclaimer: I generally like a more neutral sound signature with a slight boost to the bass and a somewhat warm lower midrange. Given that preference, I will do my best to be objective in my listening and try to keep most of my own bias out of this write up (no promises). I purchased these with my own money from Monoprice for the purpose of this write up; I am in no way receiving anything from Monoprice for my opinions.
First impressions are that these are fairly similar to the iSINE10 that I owned months ago. As I have not owned them since February, I cannot do a direct comparison. Rather, I will do my best to compare them from memory.
Bass: Pretty fast, tight, and extends deep. Quantity is a little north of neutral.
Midrange: Lower midrange is quite warm; a few dB over neutral. I personally quite like the warmth and detail here. Upper midrange goes downhill quickly for me; The upper midrange and lower treble are heavily scooped out and lacking clarity. Female vocals in particular sound veiled. I would consider it slightly worse than the iSINE10 which was also lacking in this area.
Treble: Detail is very good for an IEM in this price range. However, the treble sounds heavily rolled off and somewhat distant. The good news here is that there is no chance of listening fatigue or sibilance.
Soundstage/Imaging: Soundstage is among the widest I've heard from an IEM. This is probably helped by the fact that it is open-back. Imaging on the other hand is just average.
Comfort: While the feel of the M300 is fairly similar to the iSINE, I find the M300 to be less comfortable. For reference, I didn't find the iSINE to be very comfortable to begin with. I usually use large tips for IEMs and even these small tips don't fit my ear very well; all the tips are quite large. I experimented with the different tips and ear hooks and nothing seemed to fit "just right". My ears are a little different from each other and I found the left earpiece to really hurt the antihelix of my left ear. So much so that I really don't want to wear them for more than a few minutes at a time.
Cable/Ergonomics: The cable seems to be lifted directly from the first gen M1060. It is quite thick and a little on the heavy side for an IEM cable. It is also not very supple and likes to retain its shape from packaging. The earpieces themselves are also quite large (obviously) and slightly on the heavy side. However, the size and weight do not seem to be much of an issue for me as the over-ear ear hooks easily hold them in place; I do not feel like they would fall out.
Conclusion/Other thoughts: The inner box was upside down when compared to the outer box (see picture). Not an issue, someone just put it together upside down. The magnet array in each earpiece is very strong; they love to stick to each other. IMO, these are not worth buying. There are better-sounding and (much) more comfortable options for the price. I will be returning my pair to Monoprice.