I do have custom, they do tends to be very comfortable and detailed. BUT, they will never reached the sound stage and bass response of an proper FULL size headphone. They can be close, but still falls behind. I think the only advantage of an CIEM over an OPEN headphone is the seal and the fact it is easy to drive.
If you want to spends $1000 dollar on a earphone and try to get the best sound possible with possible outside applications, by all means, get the CIEM. They can reach 80% of their potential by just plug into any decent mp3 players. And also the perfect seal gives you very good isolation and comfort. If you only listen to music at home with a quiet environment, HD800 is a better to go since they do offer you much much more in terms of sound. (But you have to have a deep pocket, since you need at least $2000 to get a decent DAC+AMP combo system to drive it well).
For me, it is safer to go with T1 since it is cheaper and easier to drive. And AKG K702 also shares similar signature of HD800.
Bottom line, in headfi, normally you pay for what you get, but the more you pay, the less you get~~~~~