The fabric usually is really bad. I like my clothing to breathe and not have static, something that polyester and other cheap blends are not known for! Unfortunately that has been my experience with Forever 21.
They have good basics if you are looking for those cotton/spandex jersey type basics and don't want to spend a lot. But other than that the styles I find are too "cute" in an infantile way.
Every once in a blue moon I will find something I like from there. I bought several striped oxford button downs from the mens section and those have held up well.
But whatever you do, DO NOT go there on the weekends, seems like every store I've been too has been full of bratty loud pre-teens with their mothers. Too much commotion and the store is overpicked.
Also, the company's owners are uber-religious. They even print a bible passage on the bottom of their shopping bags. All this religious-right posturing and they still use sweatshops....hmm hypocrite much?
Anyway, good: super cheap, fast fashion.
Bad: Proselytizing, sweatshop labor, shoddy workmanship.