It depends what you want to use the site for.
I'm new at writing, and I find writing.com very valuable. I use the site's numerous contest prompts as an incentive to write more, and I constantly receive valuable, detailed reviews on my stories. I also started my first blog on that site, to experiment and see how it works.
I found a mentor, joined groups, won contests, received and gave reviews, subscribed to weekly newsletters (one of my stories was displayed as a link in one of them, and sent many readers my way!), took a class, read articles about writing, won a raffle, visited a chatroom once or twice...
It works for me! I now have about thirty stories I wouldn't have written otherwise.
If you are an experienced writer looking for publication, it may not be a site you would like. But for beginners like me, it's perfect! I found a handful of professional, published writers that were very helpful in reviewing my stories and giving me hints on how to improve them.
One contest I particularly like is the Daily Flash Fiction. There is a different prompt every day, and you have 24 hours to come up with a story of 300 words or less. There are usually less than ten participants, so there is a good chance to win. I received many reviews on the stories I posted there, because they are so short.
Of course, if you just post stories there and wait for reviews, you may be disappointed. You have to get involved, give reviews, post in forums and contests. If you review other writers' stories, they often return the favor.
I hope this helps!
