As far as we know, PGP and GPG are completely unbreakable, even with the full resources of the NSA on your case. We've also had reports that it doesn't work. One report, though, you should be aware of. One person has tried using Mailcrypt on Windows NT, with poor results. This is certainly no joke.
Many workstations carelessly share their whole filesystems across the network.
Many are shared by multiple users. Temp directories are usually world-readable. This means that someone might learn your passphrase if you use Mailcrypt carelessly on an NT workstation. Only use Mailcrypt on a workstation over which you posess sole physical control. Tightly restrict network file-sharing, and clean your disk often with a secure wiping utility.