I've never heard the RE-0s. But I bought S4s and MC3s (MC5s with iPhone mic) to compare to use with my phone, and ended up returning the MC3s.
S4s have a ton of bass. Way too much actually for my taste, and the low-mids tends to be a bit muddy, too. EQing the low frequencies down makes them sound quite decent, though. Of course, if you like a lot of bass, this may not be a problem.The highs can be harsh at first, but they calm down after a couple of days of non-stop burn in.
The main reason I returned the MC3s was its mids, which I found very anemic and lacking. Listening to AC/DC, I was almost shocked at how nasal and honky Angus's guitar sounded. A Gibson SG through a Marshal stack (or even a half-stack) does not sound like that at all. Getting a good fit was a huge problem for me, but even when I would force the pieces to stay in a position to get the best sound possible by holding them, the mids were way too lifeless to my ears. Bass is subdued, but the timbre is quite nice. Highs are definitely there.
The S4s have a bit more "air" to the sound. I can hear the after-attack on the drums, kind of a reverberation, on the S4s. The drums on MC3s were extremely dry sounding.
S4s tend to give you the music as a chunk rather than individual instruments. Instrument separation is far better on the MC3s.
The S4s are hands down more comfortable to wear. MC3s hurt my ears after about 30 minutes. This of course depends on the individual.
The S4s are not great. Far from it. But they were more acceptable to me than the MC3s -- I can live with their shortcomings, whereas I felt the weakness of the MC3s would drive me nuts. Of course, YMMV.