Pros
Ning is an awesome company with smart and friendly people. The interview process is rigorous so you know you're going to be surrounded by the sharp ones. The environment is relaxed and fun, working from home is always possible. The office provides a wide variety of beverages and snacks. Pretty much anything you can think of that whole foods would have, Ning has an unlimited supply of it. There's also free lunch daily. Also, working in the middle of downtown Palo Alto is pretty nice. Most importantly, you get the opportunity to work on an awesome product with a lot of great potential to become something big.
Cons
Ning can be a little bit stressful at times. Working nights and weekends can be expected of you if there's a deadline approaching or a release.