At KLM, custom 'moulded' Elacin earplugs are standard issue to any pilots, apron staff and cabin crew who want them. Apron staff tend to wear them under their ear defenders, cabin crew wear them during the cabin service (especially on the turboprops), and even pilots who work in the less noisy cockpit types also wear them.
Basically they take a wax (no pun intended) mould of your ear canal (or whatever its called) and manufacture a set of ear plugs that are in theory, a perfect fit for you. They even mark them L and R to help you differentiate
I wear them myself most times whenever I am using the on board telex headsets (not noise cancelling) and find them to be excellent at cutting out backgound noise whilst still allowing me to hear the RT and normal conversation in the cockpit. A lot of my colleagues also wear them, and are happy with them.
A number of colleagues do however find that they get uncomfortable after a while. I suspect that this is a result of poor fitting or moulding of the ear plugs.
Whenever I have tried them in conjunction with noise cancelling headsets, I have found that the earplugs and the headsets themselves cut out nearly ALL sound, so I would not advise that particular combination.