I've been using this software for about 5 months. It has solved my security worries in an easy way. As I carry a MBP with sensitive data, I had no other choice, but to use FileVault (FV). The problem with that is that FV encrypts your entire home folder, with the big speed compromise. In case you use app with intensive harddisk use, you will get into troubles (for example GarageBand or FinalCut). You could move those files outside your home directory, but that means reorganize the way I use my computer, and I did not want that.
Espionage lets you encrypt only the folders you want in the same way FV does (encrypted Apple disk images) in a way you usually wont even notice there are encrypted. It also comes with some application templates to help you encrypt, for example, all your Mail in a single step (it knows the folders needed to be encrypted and selects them automatically).
The other VERY important thing is with backups. Espionage comes with some integration with TimeMachine , to block the backup of the protected folders and provide a more efficient way to backup them (More info about this on their web page). For now, it has been good enough for me.
It also works with Dropbox.
Support it's also very good and fast.
Recommended in case you need to encrypt some folders and can't compromise the loose of harddrive speed of your entire home volume. Is this is not your case, FV could be just fine.