Well, it's not like there isn't a million posts about using that hook or anything.
And yeah, firestats was horrid. I mean crippling.
Let me recap a bit on it.
The front side of it (actual collection) wasn't too bad, although it was noticeable.
It was the backend that killed it.
First had it on a smaller box, the same as the MU install (private, dedicated, nothing else on it). Nothing special, a little slow. Then again, so were other things.
Moved the MU site in question to a new box, again for its sole use. And it was pretty beefy (quad core, 4G RAM, etc). MU: ran like a champ. The firestats back end... not so much. I had it set to refresh stats in the back (while viewing) at 5 minutes. The refresh would never finish before the next one set in. Doubled it, same result.
Moved firestats to its own server, a dual core with 2G of RAM. Nothing else on it. It crippled that box as well. This was only with a few blogs using it, and me coordinating backend usage to where there were 3 people viewing at once.
I couldn't imagine releasing it to the full site for use. It would have rendered the site dead in the water. Heck, it already was for all intensive purposes.
I didn't dig into the code, but my best "guess" (which they're usually pretty good) is a combination of bad db structure/layout and trying to make code "too fancy", for lack of a better term.
If you're running your own server, there are plenty of server level programs that are much better. Of course your users can't view stats, typically, but that's OK.