There are different types.
IE8 - the biggest sense of distance, not closed in, but everything's far. Near is far, far is far, and while there's good separation, there's not enough depth left over to really show layers, there is no sense of proximity, intimacy, a very theater kind of effect where you're always up in the balcony hearing whatever you're listening to on stage.
Custom 3 - spacious with good linearity of size, close is close, far is far. As far as creating a well spaced sound stage, excellent locational sense, layering, and separation, this is one of the best examples I've used.
SA6 - open, airy sense, but there isn't much in terms of putting a sound at a virtual location, at least not in a pinpoint sense. There is the benefit of high transparency where the source disappears and the sound is just sort of there. it's a good quality worth mentioning and not as common as you'd think for most earphones
UM3X - the opposite of the IE8, puts the singer right in front of your nose, almost claustrophobic to some people, but depth is excellent with precise locations of sound, layering, pretty much could break out the tape measure and measure how far away every sound is, but...it starts off uncomfortably close that's off-putting for some, other earphones are more naturally distanced.
etc...
etc...
I see an earphone with an excellent sound stage to have a certain mix of qualities.
1) a natural sense of space, not too close or too far
2) excellent transparency where the earphone is not heard, just the sound
3) able to place sounds in pinpoint locations, the correct locations
4) limitless depth, should not sound boxed-in. Something should be perceived as sounding 100 feet away if it is supposed to be 100 feet away.
5) limitless proximity, there should be an ability for sounds to be perceived as if it is an inch from your ear
7) a sense of space, area, the ability to provide enough ambient detail to recreate the recording area
8) if right, it should be good enough to fool you into believing a reproduced sound is a sound happening in the real word (ex. talking in the recording makes you think a roommate is talking to you in real life and you take the earphone off to see what he/she wants)
There really isn't much out there that does everything well. Certain earphones do this or that great or a few things great, but most don't really pull everything off to get that "it's like I'm there" kind of presentation. There's almost always one shortcoming or another that limits the immersion and illusion. In terms of IEMs specifically, the Custom 3 has been my favorite and does the most stuff right.
