They can grab all your passwords, because you're going to type them into their service, and because it's a web service, they can switch the implementation as th

They can grab all your passwords, because you're going to type them into their service, and because it's a web service, they can switch the implementation as they please. If it's implemented as they seem to imply (server just sends you an AES-crypted blob of passwords, JavaScript AES implementation decrypts the blob client-side using the packing key), then what happens if the bad guys root them and "enhance" their JavaScript to send the packing key back to the server? I think that you are going to have to trust such a service with your plaintext passwords, because you need to recover the plaintext passwords from it. The problem is aggravated by it being a web service whose implementation can be switched at any time.