I’ve used the Twitter Bootstrap toolkit on a number of prototypes since it’s come out and I’ve gotta say that I feel it was really well done and serves it’s purpose fantastically. Twitter really did the community a great service in releasing this project to the public.
I’m sure you’ve seen just as much as I have that some projects have actually made it to production using Bootstrap. Even though it’s designed to be generic enough to accommodate your Web app, the aesthetic is identifying pretty quickly.
If you’d like to make your Bootstrap powered prototype a bit more unique right out of the gate (without messing with LESS), check out Twitter Bootstrap Generator, a one page form that lets you customize the look of your Bootstrap with the click of a button.