Reminds me of the old Domino magazine's user section or a really graphic delicious.
It has its uses (especially in design inspiration, haircut pix and historical collections) but some people just pin all day long. It's like any internet addiction. I would rather sew or play a video game because those aren't at all addictive. Ha.
The one bad thing about the death of Domino: You lost your files when they closed. So, please, go forth and proselytize pinterest to keep them open!
The only negative I know about is the sign up process links either facebook or twitter. If you're addicted to one or both, it's not a problem. If you hate them or don't have them, you won't be pleased.