Differentiate between
Protection and
Encryption of a Workbook. Encryption using AES-128 is bloody strong, while protection is pretty weak - MarcJF is quite right about that.
There's a useful article on the MS website
here, relating to Office 2003.
Bear mind that any encryption is only as strong as the weakest link - usually weak passwords and poor key management (e.g. emailing the password in plain text).
SD