Pros: Noise isolation. It's like wearing hearos earplugs.
Cons: The wires are really stiff, sound isn't anywhere close to a similar priced over the ear headphone
I use these when exercising next to cars. I used to use sealed closed back headphones which didn't block out noise well, but these definitely do. When outside in a loud environment and wearing these, I set the MP3 player's volume to the same level I use when listening to these IEMs in a quiet room. The isolation is that good (I use comply sleeves, but the sleeves included were also pretty decent).
However, I was definitely disappointed by the sound quality, relative to a $300 over the ear headphone. I will never choose this over a headphone if listening to music in a quiet environment. The selling point on these isn't sound quality, but rather noise isolation. I won't take more than 1/2 point off for sound, though, because I imagine all IEMs are like this and that this is good IEM sound for $300.
One other problem is that the wires are really stiff, a design feature I don't understand. Lastly, putting these on isn't as simple as putting on a pair of regular headphones, you'll have to spend 15 seconds or so seating them correctly to get a proper seal.