I read this thread last week and picked up the M4 w/ mic on Amazon for $28. I just opened the package 20 minutes ago. I wanted something cheap that I could throw in the bag and use on the go with my iPhone. I've been using $300+ IEMs during my commute and wanted something that required less physical maintenance and wouldn't have to worry about losing/destroying them.
These are very early impressions (only listened to a few songs) but I find that the first minutes allow me to hear differences with new phones comparing to my existing phones before I get acclimated to the new signature. The sound is definitely U shaped as everyone said. Depending on the song, some can sound really wonky, but it works well with most of what I listen to so no problem there.
Using an amp (Pico Slim) improves the clarity overall and adds a little sparkle to the mids. For fun, I added a 75 ohm resistor (Ety P-to-S cable) to the mix, and the mids became extremely recessed -- bad idea.
Anyhoo, for the price, the M4 works for me -- cheap IEM with mic and decent sound quality.
Edited by socrates63 - 2/20/13 at 1:29pm