IMHO - There are few disadvantages to turning it off. I avoid FileVault because it:
1. does not play well with backups (because it is effectively a single file, and thus you wind up recopying your entire home directory if you change one small file)
2. appears to have a higher risk of failure than I like
3. is over-the-top for most people (i.e 99.9% of the stuff it encrypts does not need to be encrypted!)
If you have documents or materials that require encryption, a secure disk image is very capable, and hard to crack. Or, you can use a program like TrueCrypt, which even hides the existence of the secure data.
These solutions won't encrypt your email, but your email will be unencrypted anyway on your ISP/email provider servers. If you want to encrypt your email, the best bet it to get a digital certificate and sign/encrypt the secure email (this requires that the recipient also have a digital certificate, and has the upside that your email is encrypted on the email servers too).
If you require top grade security, you can try a commercial product to encrypt your entire hard drive (the encryption is unlocked via password in the pre-OS boot stage).