I just received my pair a few days ago, and am pleased with these iem's so far. The build quality is very nice, and fit/comfort is down right fantastic with the right tip (I'm using the smallest sized spin-fit tip). The iem almost "disappears" after about 10 minutes of listening, and it's actually more comfortable than the CIEM's I have because I can move my jaw, and smile while wearing them without discomfort. Go figure. These will be great for traveling and long flights.
Regarding sound quality... I haven't spent enough time with it to make any definitive claims, but I think that people will feel they got a good deal for what they paid for. I have them paired with an AK70 MKII using the optional 2.5mm trrs balanced cable. The sound is nicely balanced, slightly on the "fun" side. Resolution is pleasantly surprising, treble is crisp and airy, but the mid-bass can get slightly grainy & flabby when pushed hard with some songs. Overall I think it is a very solid offering in the $300 range. It's not TOTL material, but it's about 80-85% there; and they hit it out of the park on the comfort side. Nice job Massdrop.