I used to be a big fan of RE0, but recently I stopped using them altogether mostly because of terrible build quality on this IEM - the sound constantly stops working in one or both channels and the cable is prone to hardening from exposure to sweat. However, I also tend to favor the sharper, better defined and vivid sound of good single balance armatures these days and thus listen to Phonak PFE (single driver), or Etymotic HF and ER4 instead of RE0. I find the Etys or Phonaks do pretty much everything that RE0 can do, but offer better speed, separation, clarity and a more precise imaging which to my ears provides a more realistic listening experience overall.
I highly recommend Etymotic HF (2, 3, or 5 doesn't matter since they are all identical sounding) first and Audeo PFE second if you are an analytical listener looking for an IEM on a budget. HF is more controlled than PFE with better imaging, higher resolution, especially in the bass and midrange and more natural timbre. PFE is better if you prefer a bit more bass than a neutral response of the Etymotic provides and more impact to it while sacrificing a bit of control and definition. PFE is also more dynamic than HF5 and perhaps a tad smoother as well in the lows and mids, although HF5 has smoother and more linear highs. HF5 doesn't really have any flaws to my ears though, while PFE has a noticeable resonance peak in the treble than can make it sound somewhat edgy and harsh, especially with the grey filters. Both are preferable to RE0 for analytical listening IMO as RE0 tends to round off the edges of sounds more and is not as precise as the balanced armature based IEMs.
