Mailvelope is a browser extension that is described as the easiest way for mere mortals to send PGP-encrypted messages. Researchers at Brigham Young University brought in a group of people unfamiliar with Mailvelope and observed them try to install it and use it to send an encrypted email. Almost everyone was unable to do it. The researchers concluded that "modern PGP tools are still unusable for the masses."
From the study:
In our study of 20 participants, grouped into 10 pairs of participants who attempted to exchange encrypted email, only one pair was able to successfully complete the assigned tasks using Mailvelope. All other participants were unable to complete the assigned task in the one hour allotted to the study. This demonstrates that encrypting email with PGP, as implemented in Mailvelope, is still unusable for the masses.