So comprehensive...and yet no mention of the Stax S-001 or S-002 systems (or the SR-003 standalone IEM set meant to plug into Pro bias Stax desktop amps), argh. (Then again, those kind of stretch the definition of "IEM" a bit...you'd understand when you see them. Much bigger than any other IEM sets I know of.)
You wouldn't need to look any further than this particular guide if MLE and I could sort out the amplification issue for the latter.
As for IEMs, all I know is that the EDGE Acoustics GX-400 is a rare case of an IEM marketed specifically for gaming, and that the in-line mic and volume adjustment is literally the only thing differentiating it from the Etymotic ER-4P it's based on.
Of course, there's also those Stax IEM setups I mentioned earlier, which have a reputation for soundstage well beyond most IEMs. As for how it compares to full-size headphones, I can't say yet without an audition.
The only thing that kills the appeal of those "earspeaker" designs is that the natural crossfeed combined with the drivers being perpendicular instead of parallel to your ears would make any binaural/HRTF-mixed sources utterly useless. That means no Dolby Headphone or CMSS-3D Headphone for me, and that just won't do.