ER-4 with custom molds here. Customs give you almost complete isolation but there is a warming-up period. They actually hurt for the first week or two that you wear them. Recommended if you can put them in and keep them in but not if you must be interrupted regularly.

One more thing to be aware of with Etymotic products: the microphonics. Lots of noise from the cable hitting stuff. It looks super dorky but the cable clip fixes a lot of that.

Another etymotic ER-4S user here. They lasted two years and, when they broke, they were 3 weeks out of warranty. Etymotic replaced them for free - good customer service.

+1 happy with my ER-4S, but they are probably not worth it compared to those (which weren't available at the time).