I will vote for Johnson if he starts polling 25%. At that point, he could have enough momentum to conceivably take the White House on election day in a three-way race.
In other words, I won't be voting for Gary Johnson. I understand the concept, having voted third-party in the past, but I don't see what good it'll do for the country in this election. Too much is at stake, and contrary to what many libertarians claim, the differences between Obama and Romney are vast. Sorry if I come across as a douche on the subject...that's just my opinion.