Just spent a few hours on a Sunday playing with the latest release of Zurb's Foundation framework. I used it to create this new theme for the site, which I've called "Futura Green" - for obvious reasons.
I'm pretty impressed with Foundation, it seems like slightly more of a blank slate than Bootstrap, which can be a bit of a pain to skin up, and obviously there is far more going on in terms of pre-built tools than you would get with H5BP. I make use of Bootstrap and H5BP frequently for work, and I will definitely be adding Foundation into the mix from now on.
I was particularly interested in the grid component, because I've been working on responsive grids for a couple of my clients recently, and I like most of what I see within the Foundation grid system. I can certainly see myself taking some of its facets into my own custom frameworks and client projects.
That said, I think that there may be some issues surrounding the use of ems and the grid's .row class which make it difficult to change the font size of a page section containing multiple grid rows without messing up the layout. This seems like an elementary problem which the guys and girls at Foundation must have run into, so perhaps I'm just making a n00b error because I haven't used the framework before.