It took a little bit of getting used to, but once I did, its been great. I don't always use it, sometimes I'm just not sitting straight at my desk and its seems like effort, but when I'm typing heavily or coding, I typically do. It makes typing very pleasant.

I didn't use it to toggle Fn, but to shift it (just in case it wasn't obvious), since keeping my foot on it is really easy and letting go is the quickest way to switch back.