[Annirak]

I, personally, find IEMs to allow me to listen to music quieter than circumaurals, but I had some significant negative side effects with my Shure E3C's, which I haven't seen discussed much.  Don't try to eat with them in.  Don't yawn.  The changes in the shape of your ear canal doing either of these will can cause loud popping noises.  Any motion which causes rubbing or tapping on the cord will cause those vibrations to travel into the IEM and reproduce the sound in your ear with not insignificant volume.  I also found that my ears got tired or sore rather quickly with the E3C's.  This might be due to the style of the ear inserts.  It might be better with the molded version (another $100-200 on top of the IEM).  My E3C's came with foamies, clear PVC and grey PVC ear inserts.  The foamies were the most comfortable, but they also wore out quickly.
 
I just bought a pair of Shure SRH750DJ's.  They sound better than the E3C's did.  They don't cancel out anywhere near as much noise.
 
For me, it's a tradeoff; comfort and quality for noise cancellation.