I have the ER6i's (and several different canal phones, including the Creative EP630's ... you might consider those, as well -- they're cheaper), and it's pretty much as bazmonkey helpfully "regurgitated."
The isolation (and detail) with the Etymotics is very impressive, but you won't get the boomy bass that some listeners equate with high-quality audio. The Etys are tiny and fit pretty far within the ear -- I've used flanges, Ety foams and Comply foams, and prefer the Comply's ... but everyone's ears and preferences are different.
When, how and in what environments do you listen to music? A lot would depend on that, and your expectations.
The Sennheisers would no doubt be a "fun" everyday canal phone that doesn't offer the sort of isolation the Etys do, but that would be simple to insert and take out repeatedly. I use canal phones in the office, when I'm supposed to be available and responsive; I use the Etys to "tune out" on airplanes and would use them on public transportation if I didn't drive to work. IEMs really shine in noisy environments ... not that they're not good in quiet settings.
Both the IEMs and canal phones (not earbuds such as those that come bundled with players) would prevent sound from leaking outward, a good thing to have if you're wearing them in a library or quiet environment, for example.