Unless and until there's another state makes non-competes invalid (or California reverses this century plus old public policy) it's hard to say just how important it is, but I think it's very important, at times clearly overwhelmingly important.

The beginning of the silicon era of Silicon Valley simply wouldn't have happened without the exodus of the Traitorous Eight from Shockley Semiconductor followed by the exodus from Fairchild Semiconductor, and following that we've heard a lot about how cross-fertilization helped the industry move so quickly.

On the other hand, I've personally experienced several startups that never got off the ground in Massachusetts and Virginia because one or more of the potential founders were restricted by a non-compete. In one case, a bunch of us just all did something else before getting back together.