I bought these primarily for monitoring when recording cello, since traditional tuning pegs don't go well with big over-ear headphones, and they work great for that! Good isolation, and I can really hear my monitor mix without much interferens from the direct sound, if that's what I want.
Side note: Also tried them with violin, but that turned out to be a surprisingly bad idea! Plugging up the ear canal greatly boosts the vibrations from the instrument through the yaw bone, which is terrible if you actually want to hear the monitor mix. All I hear is boom, worthy of a huge viola! Not the fault of the product; I suspect this is just the physics of in-ear monitoring.