I'm therefore probably a bit biased on some of these questions!
Brooke on a girl is fine. I think it's one of those names that works equally well for both sexes. There tends to be a bit of a blonde/bitchy/cheerleader stereotype for the name on girls (Brooke Hogan doesn't really help) but I like it. I do think it sounds better in NZ/Australian accent than in general in the US accent (I know there's more than one US accent, but the vowel in 'Brooke' is fairly similar in most I think?). That's probably just because I'm more used to hearing it the Kiwi way though.
It probably is a bit dated, but I think because it was never really uber-popular (in the way Jennifer was, or Heather), it's doesn't sound AS dated. I think it's still fine to use now (and is still being used).
I don't know any Brookes as such, but there are a few I've met or know of (all girls): one my age (21), one a couple of years older, one 2 or 3 years younger, and one 4 or 5 years younger. I don't think any of them spelt it Brook, that tends to be seen as the 'masculine' spelling. There's also a (male) TV presenter in NZ called Brooke Howard-Smith.
Don't know any other middle names, but mine's Rachel. I think it works quite well.
As for it being common, I don't think it was ever uber popular here, but it pops up in the newspaper BAs occasionally. I think it's sort of just floating along. Compared with Jenna (I've met or know 5 or 6 the same age as me, but never met one younger or older) which was popular around the same time, I think Brooke is far less common than it is generally perceived to be.
Wow it's weird seeing my name like that. Somewhere in there it lost all meaning and I found myself wondering 'Is that really my name?' I think it might be time for lunch! :-P