Etymotic invented the in-ear earphone in 1984 for use in diagnostic testing and auditory research. The first consumer model, the ER-4, debuted in 1991. I remember that when I heard it the resolution of fine detail was far ahead of any dynamic headphone I'd heard at that time. The ER-4 earphones are still in the line. I still use my ER-4P and love it.
The MC5 is a new model, and priced at $79, it is the most affordable in-ear design ever offered by Etymotic. The company claims its headphones produce superior isolation from external noise, and I agree. The MC5 did a better-than-average job blocking out the sounds of the NYC subway. Etymotic headphones tend to sound more accurate (or less hyped) than most under-$100 in-ear headphones. I didn't have any budget in-ears on hand to compare them against, but the MC5's sound was more naturally balanced than Monster's standard Turbine in-ear ($150). That was especially obvious with acoustic music like the Avett Brothers "Emotionalism" album. The brothers' soaring harmonies and chiming guitars really shined over the MC5; the Turbine sounded more immediate and brighter, but lacked the MC5's warmth.
Gil Scott-Heron's "I'm New Here" album's sound mix, with its buzzy keyboards and rumbling basslines sounded fabulous over the MC5, though the Turbines had deeper bass that made more of an impact, but its more upfront balance added an edge that detracted from the music. They are very different-sounding headphones, and, as always, selecting a winner is very much a matter of personal taste. Etymotic's MC3 ($99) headset model offers the same sound as the MC5, but adds a microphone and volume controls for iPods.
Etymotic also sent along the HF5 in-ear headphones ($149), which feature a metal body and a more upscale appearance. The differences are more than cosmetic; the HF5 features a "balanced armature" transducer, similar to the one used in Etymotic's flagship ER-4 in-ear headphone (the MC5 uses less expensive moving-coil driver technology). The HF5's bass nearly matches the Turbine's oomph, and sounds clearer and more refined than the MC5 or Turbine.
Regarding durability issues, both Etymotic headphones seem about average in build quality. I wouldn't blame anyone for hesitating to invest in quality headphones if they've had headphones break after just a few months of use, but those headphones probably didn't come with the protection of Etymotic's two-year warranty. Of course, the warranty won't cover outright abuse, but short of that Etymotic will repair or replace the headphones within the warranty period.
Both Etymotic in-ear headphones are designed to be used with their eartips inserted deeper into the ear canal than the Monster Turbine's and many other in-ear headphones. Deep insertion is part of the reason Etymotics provide superior isolation from external noise. Failure to fully insert the eartips will allow more noise to come through and result in a lack of bass; so if you're squeamish about putting things into your ear canals then Etymotic headphones might not be right for you.