My thanks to those of you who recommended the ER-6i for my application. I got it yesterday and spent a couple hours trying it out as well as comparing it to my great HD-595 for home listening. Short answer, it sounds good on the kinds of music I listen to, it's decently comfortable with the 3-flange tips and it's going to work great on the airplane with my iPod Mini. I do have a few more detailed review comments...
First off, with the unamped iPod Mini the ER-6i has a fairly similar sound to the HD-595. There are two exceptions: 1) the Etymotic has a more "polite" sound that eliminates any harshness or trace of sibilance in the recording and 2) the attack and decay of bass notes are slightly better controlled with the ER-6i. Now by "fairly similar" I don't mean that the two sound signatures are difficult to tell apart. There is certainly a rounder, fuller character to the IEM sound which comes across as being more inside your head which is not a sensation I particularly enjoy. But for extended listening (such as on an eight hour plane trip) I suspect it will be very pleasant.
Plugging the ER-6i into my home-listening setup (Adcom CD player, Portaphile headphone amp) revealed the "polite" sound for what it truly is...the top end is rolled off. Now admittedly it's a gentle roll-off and on some material it even makes for easier listening at high volume but there are details revealed by the HD-595 that are either subdued or missing altogether with the ER-6i. I think with 192kpbs AAC (and even 320kpbs AAC tracks) on the iPod those details aren't there to be revealed with any headphone but my Adcom/Portaphile setup is very detailed and perhaps even accentuates the high end which is a better match to the Senn than the Ety. For my home listening, not that there would be any reason to use an IEM anyway, I like the sound of the HD-595 far better.
There's one other thing that is slightly noticable with the iPod and much moreso with the Adcom/Portaphile playing a Redbook CD. The one characteristic on which I'm the harshest judge of music reproduction equipment is upper-bass boominess, the kind that can get real annoying on acoustic guitar music (especially if the guitar accompanies female vocals). My HD-595 is as nearly perfect in that regard as anything I've heard, the upper bass will be clean as long as the recording is decent. I'd give the ER-6i a rating of "Acceptable but not Great". There is certainly a hump in that frequency range but it's a wide and gradual one and really comes just distort the timbre and balance of the music slightly rather than getting boomy covering up the other voices and pitch ranges. In other words what I find "Acceptable" is, by the standards of reasonably affordable head-fi gear, really good.