I've been working with the release candidates for a while for my Shopify-based Bootstrap theme (http://bootstrapforshopify.com/), and while it's been a bit of a tough ride sometimes (with pretty significant changes from release to release), I'm really impressed with what v3 brings to the table.

Hopefully, the way the new styles are laid out will encourage more people to treat Bootstrap as a starting point rather than the end game for layout and styling (although it's a pretty good starting point in of itself).

Edit: And, of course, massive thanks to Mark and Jacob (plus the ever-increasing community) for all of the work they've put in over the last two years :).