Is filesystem encryption any good if someone steals my computer while it's on?

While the contents of the disk are encrypted, the operating system is decrypting the content of the disk on the fly in order to access it. The screen lock does little to protect the contents of the disk while the computer is running.

All the screen lock does is prevent someone from running programs / reading your screen while you're away. As long as the computer is running and the encrypted disk mounted, your data is vulnerable. Granted most thieves would just shut the machine off / reboot it once they had it.

However, if you're paranoid about your data, the inconvenience of powering down/powering up every time you walk away for a while is a small price to pay IMHO.