I never had an obstructed view ticket, but when I was little and we had season tickets (before the prices got ridiculous) we were about 10 seats away from an obstructed view seat. It depends on where you are, but some seats are literally directly behind a 2 foot wide steel support beam. It can suck real bad but only one seat really gets screwed.
I've been in obstructed view seats at Fenway and it wasn't good at all. I barely sat in my seat the whole game cause the view sucked. Hopefully you have a better experience.