I have purchased a lot of fabric from my Joann's store. also, even more from my LQS. there is a definite difference in the quality, but that being said, some things I make, I am not concerned about the fabric as I am with other projects.
Our local quilt guild does a lot of charity work and I often purchase fabric from Joann's for these projects. But, there have been times I have purchased fabric that after washing has turned out to be really lousy. I guess it all depends on what you want it for and what you are going to do with it.