Absolutely amazing... If you can get a good seal. Those with thin heads and a prominent space between your jaw and neck right below the ear and those who wear glasses may have trouble getting a good seal and that will result in much lower bass than you would probably want. If you can get a good...
It depends on the type of format. Mp3 sound takes less space while wav takes more space.
For example: My last voiceover took 75MB as wav format, while it takes 7mb in mp3 after conversion.
I hope it helps.