I have read many articles about the benefits of using passphrases in contrast to passwords. For more details, you can read http://blogs.technet.com/robert_hensing/archive/2004/07/28/199610.aspx.
I have always been convinced about the use of passphrases. First of all, it is easier to remember even though it is very long. If I want to keep a 30-character password, It is quite easy to remember "My mother maiden name is Smith". On the other hand, I admit that my memory is not equipped to handle "ad8asdf89x$cm$nd$z3910xkd!#$*%" that needs to be changed regularly due to company policy. (Both examples are not remotely like my actual passwords. Don't even bother.)
Longer passwords are better because it is less susceptible to brute-force attacks. Passphrases are great to enable the use of long passwords. Windows supports up to 128 characters, but I am not about to have a 128-character password because it takes forever to type, though.
The downside is that not many websites support long passphrases. You still need to resort to cryptic passwords for your protection.