For basic encryption of user data OR for the hiding and protection of data you not only want to encrypt but actually wish to hide completely (i.e. make it impossible for anyone to actually know there's encrypted data on your Mac) TrueCrypt is the way to go.
It now supports 64 bit on OS X Lion.
You can create hidden volumes, which makes it possible to completely hide encrypted data. This allows for plausible deniability. Simply put, a Govt agency or some other threaten foe will not be able to force (through law or malice) you to reveal the password to your encrypted data, because there will be no way to prove this encrypted data exists. It's unlikely most people need such plausible deniability, but for those who do TrueCrypt makes it easy.
You can encrypt any volume, which includes external drives and USB sticks. A wise move if you have ANY data on your stick or drive you'd rather no one but you could access.
TrueCrypt is open source software, which means all the application code is publicly viewable. This eliminates any chance of it having "back-doors". As far as I know, Apple can not make the same claim for Filevault/Firevault2, because it uses proprietary code. Again, most people are not likely to need this added level of security (i.e. the US govt. not being able to back-door their way into your data, with Apple's "help"), but for some this may provide added peace-of-mind.