I've been told the isolation at low to mid frequencies is superior to the
best over-the-ear types.

I've found you really need a tight seal to get adequate bass from an in-ear.
The custom fit units of Westone or Sensaphonics do well, but if either leaks
then they are worse than a generic in-ear. Either of those are pricey $600+
units, and Westone has a triple driver job that costs more.

I can wear a custom, that fits well, all day. That "fits well" is the trick
and you have to be able to send them back for a re-fit if they do not seal
or hurt our ear. That can cost for another impression though, in my
experience.

I liked the Etymotic's ER-6 series for their high response and Shure for the
lower response. Koss "The Plug" was probably the lowest of all but it
sacrifices the highs too much for me and the foam gets kind'a nasty after a
while. All sound different and it depends on what you are looking for in
sound (treble vs. bass).

Personally, I love my Westone's as they fit so well, are easy to clean, and
block just about all the ambient noise that wrecks the sounds I'm listening
too. I use them beneath a motorcycle helmet which is a harsh environment
for low frequency wind noise and while camping to block out the all-night
kegger party in the next campsite. When they fit well, you get that cold
sensation when you screw them into your ear canal and they seal up.

Just gotta spring for the custom fit though.

B~