In my experience, I cannot feel the seal with the ex600s because of the vent (you don't get that under the water feeling), this might be just me though. When music is not playing through them, isolation is pretty poor, and I can hear everything around me. This is just my personal impression of the ex600s and it might be different for you.
The same impression with my half-in-ear Brainwavz Beta. Definitely a noticeable difference from proper sealing IEMs.