If it's consensual then I don't really care either way, in my opinion people should be allowed to make up their own mind with what they think is right or not. Just like a gay relationship, it's not my thing, but I'm not against it either.
There is, however, a difference when it comes to gay marriage and incest. I support gay marriage, but I wouldn't actively support incest, for the simple reason that it has a negative effect on the human genepool, just like any other form of inbreeding. I do think we have the responsibility to ensure the survival of the human race as well, but considering the (expected) small percentage of relationships, I consider it a minor issue in this case. I don't think that I'd marry a woman who is a product of a direct blood relative relationship though. I simply don't want to run any increased risk for my children to suffer from genetic defuncts.
To clarify, for me incest is the direct bloodline relationship of a brother - sister, father - daughter, or son - mother and between those the brother - sister are seen as the worst by me, since they don't exchange any new genes whatsover. With parent - child the new child would at least still be made up out of 25% 'new' genes on average. (25% indirectly from the parent and 50% directly from that parent).
I don't view niece - nephew relationships as (direct) incest, though I still wouldn't want to marry a partner if I knew their parents had a family history together.
One big reason to be against incest for me is because it's often based on a nonconsensual relationship. I don't support rape or coercion and those guilty of such should be prosecuted.