I bought these headphones a few months ago for my 45 minute daily ride on BART (Bay Area Rapid Transit). They took a few days to get used to, but after that they were awesome.
First off, this is my second set of fairly expensive headphones. My first was the Sennheiser HD-555. Those are nice, but they are a completely different class of headphones. They are open air, and allow sound in and out. This gives them a great soundstage in a quiet area, but makes them ineffective for times when you want to block sound out.
These headphones, on the other hand, are designed exactly to block sound out. They come with multiple silicon tips as well as a set of foam tips so that you can get a solid fit to block outside sounds, as well as stop sound from exiting your headphones. So if you live in a dorm, or want to use these in the library, you don't need to worry about sound leakage disturbing nearby people.
About the multiple tips, that is one of the pains of these. I tried a few different ones over a few days before I found a comfortable set. My other complaint is the wire, which is designed to bend and hold a shape. The wire near the actual drivers is designed to curve around your ear, but I am not sure why the entire cord is made of the same material. It's not a huge problem, but it's a pain when I unwrap the cord from my ipod.
But those are minor gripes because these headphones sound great. They have dual drivers, which means that each piece has both a low-range and a high-range driver to produce an accurate sound. The low-end on these are amazing, and I have been catching up on the hip-hop I used to listen to in the 90's. Snoop Dogg never sounded this good before!
If you need a pair of noise canceling headphones, and can stand the price tag, I highly recommend these.
Edit: I bought these headphones over a year ago, and I've had to replace the cord 4 times now. All three cords have fallen apart near the headphone jack connector. UE has been kind enough to replace them for me (so far) but I certainly would not purchase these again knowing the shoddy quality that goes into them. Maybe this is a conspiracy theory, but it seems like the many bad reviews starting showing up around the time Logitec bought the company. I don't know if there was a change in manufacturing, but I probably would not buy again.