For the longest time, I've flip-flopped on whether to check the 'Use all F1, F2, etc. keys as standard function keys' option in the Keyboard System Preference pane.
I love using the volume up and down keys on the keyboard, and probably use them fifty times a day. I sometimes like using the 'Dashboard' key too. I rarely use the media control keys. I'm undecided on the F1/F2 keys, though... I would like to use them as brightness control on my MacBook Air keyboard, but when my Air is plugged into my external monitor and keyboard, I want to use those as F1 and F2 (especially for code folding in TextMate).
Lucky for me, I found this great bit of freeware: FunctionFlip.
It's a System Preference pane that lets you choose which F keys are used as standard function keys, and which ones are used for the marked purposes (brightness, volume, etc.). AND, you can even set things different depending on what keyboard you're using. Nice!