I had a pair of Etymotic ER-6's before splurging on Custom 3's. I was very tempted to go for Ety's ER-4p because of the reviews they get, but I've always really enjoyed the Klipsch sound, plus the Custom 3's have dual drivers, so I went Klipsch. The sound quality of the C 3's is simply worlds above the ER-6s. I've also had Shure's E3c's. I mean, really no comparison. after the initial burn in period, I am amazed to have a product in my ears that sounds this good. However, there are some things to keep in mind.
The fit on these things is unlike any other IEM i've ever tried. it could just be my ear shape, but i've never had such a hard time fitting. i still can't make these things comfortable in my ears.
the cable issue, as pointed out in an earlier post, is something to look out for. you can prevent the cable from getting permanent kinks, but it takes A LOT of active monitoring and care. Kind of annoying...