I had been using (and getting addicted to) Function Key Mapper for several years and didn't know about One Key until it failed to work with Snow Leopard. I just thought I would give this a shot. This is superior in every way except one. One Key does not seem to have the ability to use modifier keystrokes to launch applications. So once you have assigned an Fkey to one application, that is all you can use it for. Try using a modifier and you get an error tone. Function Keys Mapper allows you to use modifiers.

However, One Key is much faster and doesn't use up any processor time to work properly (Function Keys Mapper always used up way too much percentage of the processors' time in my opinion).

I recommend it highly.