Just remember to not use the Web UI: if you do so, your plaintext password is sent to the servers, because decryption happens on the servers. This is mentioned in their FAQ. Unless things have changed since the last time I checked.
Is it feasible to do the decryption in JS? LastPass for example encrypts the passwords and never sees the plaintext. Although decrypting a plaintext password is a significantly smaller problem than decrypting large files.
Just remember to not use the Web UI: if you do so, your plaintext password is sent to the servers, because decryption happens on the servers. This is mentioned in their FAQ. Unless things have changed since the last time I checked.