The MC5 are the cheapest Etymotic phones available, and they use dynamic drivers, contrary to the often mentioned (and overly hyped) balanced armature ones, such as the ER-4 and ER-6.
It's somewhat amusing that Etymotic claims "best in class under $100"
on their website for the MC5. True to the standard Etymotic house sound, the MC5 suck every last bit of excitement, emotion, and musicality out of the sound. They make everything sound dull, lifeless, and boring, no matter how good the recording quality of the source material. Off the top of my head, I can think of about a dozen other IEMs that sound better than the MC5 in the sub-$100 class.
Everything about the MC5 is mediocre. Nothing is really overly bad with their sound, but everything is far from being good. The bass doesn't extend deep, it has little texture, sounds muddled, lacks dynamics, attack speed and punch. It has more bass quantity than other Etys, but less quality. Midrange is anemic, instrument separation isn't anything special. It lacks all the precision one is accustomed to from Etymotic's armature IEMs, it's less detailed sounding than even some $30 Hippo IEMs. Treble is so-so, it neither sparkles, nor is it recessed or rolled off. It's just frequencies, not music.
To add insult to injury, the MC5 are some of the absolutely quietest IEMs I ever heard. Even the Phonak PFE are quite a bit louder than them. This means, they're absolutely unusable with players like the Sansa Clip+ and similar. Of course amping them would be futile, since they're just no good phones - money would be better spent on another pair of IEMs. Personally, I usually listen at quieter volume levels, but the MC5 are too quiet to be useful.
Build quality is good - better than ER6 - strain reliefs and cable seem to be sturdy. Isolation is very good, up there with the SE530 and such. Two extremely stupid design decisions however make them worse than they have to be: the strain reliefs at the earbuds are 45° angled, so it's hard wearing them with the cables over the ears. Wearing them with the cables hanging down results in terrible noise, there's no reason why they didn't make them have a proper over-the-ear fit. The 3.5mm plug is 45° angled as well, which is the worst plug one could come up with. It's neither low profile 90° nor is it a straight plug. A 45° plug acts like a catch-all, hooking on everything, always being in the way. Extremely annoying. So much for Etymotic's "best-in-class fit and style" claim.
Verdict: Absolutely not recommended. Decent build quality (despite the stupid 45° angled earphone strain reliefs and 3.5mm plug), good isolation, but disappointingly anemic and boring sound, and incredibly inefficient. No clarity (for Etymotic standards), no precision, no punch, no dynamics. No music.