I've said it before, but I like Zurb Foundation because I felt that I had to customize a few things from the get-go: http://foundation.zurb.com/ (Not affiliated) And the built in responsive grid stuff is pretty nice, and you can still chose some of your own display components.
I should add that I think Twitter Bootstrap is perfect for building behind-the-firewall type of corporate/infrastructure apps. But using it bare on a public-facing marketing site?