Pros: Good amount of decent bass, clear mids and highs, good match for many music genres, uber-robust cables
Cons: Manufacturer doesn't offer silicone tips for large ears, cheap paintjob, soft plastic stripe on housings needs to be fixed with glue in summer, no bag
There are some quality issues with these phones but they are of rather cosmetical nature, like the cheap silver paint job not resistant but easily wearing off, the soft plastic stripe on the housings getting loose with warm temperatures during sommer ... I've always got some glue at hand and I don't care how things inside my ear channels look actually, at least when I'm so overly satisfied with them as I am with Sennheiser IE 6.
When it comes to built quality there are other things I'm longing for much more: cables have to be solid and shouldn't fail on me. They shouldn't turn into a mess of knots inside my trouser pocket like most manufacturer's cables tend to do. In these points IE 6 offers the highest level in quality one could ever wish.
The sound of these inears is just amazing: abyss-like deep but punchy bass responding fast enough for highly skilled technical Death Metal double-bass attacks, a (not too) warm clarity in the mids which, even in the lower segment, are very nicely separated from upper bass. And above all lay transparent and shiny highs, bringing out lead guitar solos and all of the drum's overhead stuff like a charm. I love that sounding for Metal as well as for Jazz, Jazz Rock, Rock, Blues and Classicals. These phones drive a lot of musical genres to a very enjoyable listener's experience.
People like me who have rather large outer ear channels have to overcome one ugly drawback though: even the largest silicone tips, which Sennheiser or any other manufacturer offers, are to small to get the perfect seal one needs to achive for good sound. I found a workaround that workes very good for me. Just take the largest of Sennheiser's tips, put them on the tubes and pull them all the way down towards the housings. Then grab another pair, the largest of Sony's dual compound silicones and put these on the tubes, too, let them snap into position like they're supposed to be. Then push the underlying Senn's up until they sit directly underneath the Sony's. This works perfectly well for me :D
To sum it up: In the end it's the sound that really counts. Having found a way to get them to seal my ultra-large earchannels, the Sennheiser IE 6 are a total win for me and my Cowon D2(+)'s.
I know my pattern of use, so if one day the unbelievable should happen in form of broken cable, I would go straight ahead for the IE 8 coming with replaceable cables of the same diabolic built quality!