Dave Weigel has a solid piece published at The Washington Post addressing some common myths about the Tea Party, namely that the movement is 1) not a reaction to President Obama, but to the bank bailouts; 2) racist; 3) led by Sarah Palin; 4) harmful to the GOP; and 5) going to transform American politics.
These are the claims that I've encountered routinely in talking about the Tea Party in talking to other people about it and in reading the comments sections at The Atlantic; so Weigel, I think, chose his myths well.
I would add one: that the movement doesn't actually exist, but is an invention of the media. This is a point that's been raised by more than one observer, and I think it's a myth as well. It's true that the movement's strength and numbers are difficult to quantify and that most available polling measures those who "agree with" the Tea Party, rather than those who have actually participated in a meeting or rally or subscribe to an email list, but from watching the big rallies that have unfolded across the country since the spring of 2009, it's hard to say that the movement doesn't actually exist.
As for the movement's effect on the GOP and ability to transform American politics, these hinge, to some extent, on the same question: how long can the Tea Party keep it up? It is energizing Republican voters, but if, say, the Tea Party sustains its intense anger and frustration enough to direct the GOP, while at the same time dwindling over the next couple years, it may eventually harm the GOP and transform American politics by doing so. But that's just speculation.