I've been using Zurb Foundation for a recent contracting gig, and am pretty happy with it. Can anyone who is more into these things compare it, in a thoughtful way, to Twitter Bootstrap?
Bootstrap gives you a lot more than foundation does, but I find it very restrictive, it's hard to make a bootstrap site not look like bootstrap (if anyone can point me to some counter examples I'd be interested). This is unsurprising since bootstrap was designed by twitter for a consistent look across their internal tools.
So my verdict is, for internal tools / quick hacks, use bootstrap. For mockups and real products, use foundation.
phpnode got it right - I'd be more inclined to use Bootstrap in cases for which I have no design, or a simple design with latitude for many elements. Client extranet, back-end utilities, etc.
Zurb has several other fantastic tools as well.