I'm against it.
On a biological level - potential children that are produced from it have a considerably higher rate of birth defects. And on a social level - given that often these relationships may be between say a mother/father and one of their children - there is a weird type of "authority" underlying the relationship that i'm uneasy with (similar to the relationship between a teacher and student).
There are very few things that you'll find a consensus on when you move between cultures around the world, but anti-incest attitudes is generally one of them. And i'm fine being part of that majority.