I think I paid around £50 for these, but it was a long time ago and the price has dropped- you can't buy them new anymore, so ebay is your best bet if you can stomach the idea of using used earphones (and they are definitely earphones rather than headphones!)
When these first came out (and I bought them) in ear monitors were a fairly new thing, so you either had massive 'cans' on your head or tinny earbuds- these invasive sound producers burrow deep into your ear, providing a level of natural noise isolation, and delivering sound straight into your ear canal.
At first it feels weird, the deep snug fit, but after your first 10 times you get used to it. I'd advice spending some time with the included 'fit kit' to get the best seal as a good seal is essential for best sound quality. The foam tips (squished and then inserted and left to expand) give the easiest route to good fit, but if you're frequently pulling them out then the foam tips are slow to use, get covered in sticky wax, and eventually start to look a bit disgusting- meaning you'll need to replace them often. There are multiple sets of hard silicon tips too- these are wipe clean and indestructible, but you'll have to compromise between too big or too small as you won't find a fit that's just right. Too small and you get weak sealing, and bass will suffer. Too large and your ears will suffer.
Sound quality is ok, it's quite bright and metallic, there is bass but it can lack real extension at the best of times (what did you expect from such a microscopic driver?) and with the slightest air gap in your fit the bass just disappears- you might find yourself holding these in your ears to keep the bass strong. They can sound harsh at times,
With a good fit the sound of the world melts away (watch out for passing cars) and their 'flat to the ear' design means you can lie on a pillow and not feel discomfort.
The headphone jack is a 90 degree jack, suggesting it's use for mp3 players and other devices. The included case is nice, but it's too mush effort to use so I usually just shove them in my bag- it's only after many many years of use that the cable is starting to split- when they fail I will probably search for a different product, I think the quality of IEM's has gone up over the years.
I bought these headphones a year ago now to replace a set of Etymotic ER6s that broke.
My first impressions were that the build quality on these canalphones was much superior- with more heavy-duty wiring than the flimsy wiring of the Etymotics that lead to one headphone breaking.
In use they are great- although as with all canalphones there is a learning curve present in actually fitting them to your ears. They come with an array of differently sized ear-pieces so that you can be assured of the best fit. Be warned that the foam pieces will soak up your ear-wax- leading to complaints from those who ask to borrow them!
The sound quality from my set is great, and the isolation you get from them is also very impressive. I think that the isolation is probably of function of the material that you use as the earpiece, but either way you can easily blot out the background noise on a street or in a train.
I'd really reccomend buying a set of these- I found mine on Play.com for around £25.