There are good writers and reviewers on the site. If, through the "fan" function, you build a community of your own with other writers who agree to seriously critique each others' work, it can be a good resource.
Other than that, the reviewing system encourages perfunctory and unhelpful reviews -- which you have to pay for. Here's why.
A decent review of an average story takes ten to twenty minutes. Each review awards, on average, a dollar in member bucks. So, a decent reviewer makes about four to five dollars an hour in member bucks.
In order to get your work reviewed, you have to pay at minimum $60 in member bucks, and often much more. So, back of the envelope, you need to do 60 reviews for a dollar each; you can do five of them an hour; so that's 60/5 = 12 hours of reviewing you need to do to pay for your own story to be reviewed.
That's clearly not practical.
So site members have a choice: Either churn out dozens of perfunctory and unhelpful reviews, or pay cash money, and lots of it, to get your own work (shoddily) reviewed. Doesn't sound like such a good deal now, does it?
I get that FanStory is a for-profit business, but in the balance between making money and serving writers, it clearly skews toward making money.