I come from a one-party town (San Francisco). Yes, the Green party is sort of a second party ... but they aren't nearly as powerful as the Democratic Party.
Fortunately, there are different factions in the Democratic Party of San Francisco, so for elections where Democrats run against each other (such as Mayor) voters actually have a choice. However, for some offices you do see only one candidate running, so a voter can't vote against the candidate. And if there is only one Democrat running, that candidate is going to win, period, so from my point of view it's almost as bad as only one candidate running.
And this is why I think anybody who lives in San Francisco and cares about voting should be a member of the Democratic Party, regardless of political opinion. Even in the elections where Democrats run against each other, the party has a lot of influence over who gets to run and gets the most support. Thus, the Democratic Central Committee is actually the one of the most important elections in San Francisco, and only members of the Democratic party get to vote.
That is why I am registered as a Democrat. If I didn't come from a one-party town, I would be an independent, for similar reasons as Baggins.
As far as the national Democratic and Republican Parties ... for all of the heated rhetoric, their *actions* seem to be becoming more and more alike, and not in a good way. I am disgusted to the point where, in national elections, I am probably going to start voting third party (I don't think the third parties are better ... but at least, by virtue of not having power, they are not nearly as corrupt).