I have owned two pair of excellent custom headphones, which I purchased for their sound quality (Ultimate Ears UE11 and Jerry Harvey Audio JH13s), and currently have a third on the way (Jerry Harvey Audio 3A/JH16: which is more of a system than a headphone). Prior to this, I owned Etymotic ER4Ps.

In order to get the best sound quality, the fit needs to be perfect in order to get a seal (also affecting isolation). Sometimes this is a bit of trial and error, mostly depending on the molds the audiologist makes. I had to send my UE11s back twice to get a good fit, and went through two audiologists, as the first one's molds were rejected twice (he was on their website as recommended) I was lucky on the JH13s and got a perfect fit first time, but I know of some people who had to send theirs back multiple times until right. You'd have to be willing to take that risk.

A couple other things to consider: if you're just looking for isolation, the Ety ER4Ps (not custom) offered the best isolation I've gotten, but you have to mess around with different tips. I've had the Shures, and the Ety isolation is better. If you're making the jump to customs, I would think that you would also be interested in making a jump in sq. I'd look into Jerry Harvey models, and Westone has a new one out (ES5) that is reportedly excellent (reported by people whose opinion I trust based on their experience and taste I'm familiar with). That said, the Ety ER4P provided a bit better isolation over my customs while offering quality sound, but I'd take the sq of the JH13s or JH16s 100X over the small isolation difference any day. They are pricey (there are less expensive models available), but to sit back on a plane with a simple ipod classic (you'd need to have lossless files with these more revealing headphones), and a portable amp (my preference is the pico slim by headamp), any flight becomes a joy. I'm flying from Florida to San Francisco in a couple weeks, and look forward to a few hours of closing my eyes and disappearing into my favorite music.

If you'd like more info on custom headphones, there are a couple expert sites:

head-fi.org - it's a treasure trove of genuine information that's sadly often buried in youthful nonsense and "authoritative" opinions based on little more than desire to be authoritative. Don't believe everything you read, but dig and you come up with gems of info.

head-case.org - people who know their audio, many into diy, have little/zero tolerance for the b.s. noted above. Many own custom headphones, and have had experience with most everything else. Can be a bit hard on newbies, if it's clear the newbie hasn't bothered to check things out (e.g, use search feature prior to posting away or is a general arse). Most active members know each other outside of the forum and are protective of the site. The welcome pm explains it all. Right now, there is even a current thread where options for Ety customization are discussed. If you're really interested, pm me, and I'll send you a link.

Happy listening and good luck.