I would vote for a third part candidate IF I thought he/she had a good chance of winning and IF he/she had had values/beliefs that mirror my own. National politics is a bit different. Republican and Democrat parties hold a monopoly on the bid for the White House, and even when there is a good alternative to the Republican/Democratic candidate, he can't gain a footing.
Take this election as an example. Obama is Communist/Socialist, and Mitt Romney is an East Coast Liberal who loves big government. I would love to see Gary Johnson (running on the Libertarian ticket) win the Presidency...I think his views mirror that of the majority of Americans. However, since the Democrats and Republicans control the election process, he will not be invited to the debates, and in most states will only be a write in candidate. So I guess I will once again vote for the lesser of two evils.