Works fine for me with extract, I personally love the stuff. Never brewed without it, (except for my wheat beer) and I have always had VERY Very clear beers, might get clear beers without but I was taught to use it, and if it's not broke, don't fix it. (i've had some people think I was trying to pull a fast one and pass off a commercial brew as my beer because it was "too clear to be a home brew" LOL.
That being said, I have seen plenty of less than crystal clear beers brewed by people who haven't used it in extract brews.
For the price it's worth having some on hand if for nothing else than peace of mind.