So I just recently got my RE0's to replace a pair of Klipsch X10's with a bad cable. I also got the Brainwavz M2 for free with the Nationite N2, which also included a lower level brainwavz earphone that looks alot like the M1 (that was a GREAT deal for $84). So I have been doing a lot of comparison listening over the past couple of weeks. My wife also has a pair of Phonak Audeo PFE 112's for me to compare to. I'm using the Sony X series MP3 player as my primary source, but I also have a JDS labs cMoy BB amp with a direct line out of the Sony, and a nuforce uDAC2 for laptop listening.
Overall, the RE0's have become my go to IEM. The sound is so detailed with excellent mid range and treble. The only thing that I thought was lacking is the bass. It seems to be able to represent the lower frequencies, but not in great quantities. Of course, this was after most recently using the Klipsch which have strong bass, and in comparison to the M2's which I think are a little too bass heavy. Overall, I prefer the RE0's over the Klipsch however. Next in the line would be the Phonak's. They seem a little less detailed in the upper frequencies and mid range. The RE0's do benefit from amplification, but the Sony and the Nationite can drive the RE0's fine by increasing the volume level by a few numbers in comparison to other IEM's.
In general, I am really impressed with the RE0! I feel lucky to have all these IEM's to test against each other. All of them are very good IEM's and I think it really comes down to preference, but if you really like detailed, balanced sound, the RE0 seems hard to beat. Unfortunately, I don't have the funds to test them against some of the other big names like Westone, Shure, UE, etc., but i don't feel like I'm really missing anything.