As I recommended, I think it's one of the better in-ear phones you can buy for the money. It's musically better than the Etymotic phones, but you don't get the feeling of singular clarity that the ER-6i gives you - that is, unless you have an EQ onboard your player which can cut all the bass and really push up all the trebles on the CX300 (because that's what the Etymotics sound like

).
It has acceptable isolation from outside sounds, something that the Etymotic / Shures do definitely win on. The isolation of the Etymotic (with the expensive-to-replace foam tips, and you will have to replace them on a fairly regular basis) is awesome in particular.
The CX300 is one of the better built versions of the low-end in-earphones. I thinl all that plus the two-year warranty for someone whose hard on phones would be a good deal.