Not very obstructed at all. You shouldn't miss much, if anything, depending on how it's staged. The columns are towards the back of the playing area and in most productions directors stage most, if not all of the action on the thrust part of the stage which is unobstructed and plainly visible from every seat in the house.
"What a story........ everything but the bloodhounds snappin' at her rear end." -- Birdie
[http://margochanning.broadwayworld.com/]
"The Devil Be Hittin' Me" -- Whitney