Pros
They say grass might seem greener on the other side, not always as green as it seems, and that was the perfect case with me. I was involved in the very early days of 99designs and it was a life changing experience. It's hard for me to talk about specifics, but I can only share what my wife told me about 6 months into the job. "I'm not sure what they are putting in the water there, but you are a much nicer person from almost the day you started". When I decided a change was needed about 4 years later, I must say if was a difficult few years, and when the opportunity to return to the 99designs office presented itself I jumped at the chance. I'll let others talk about the benefits, the gym, the coffee etc -- I'll just be proud of who 99designs makes me outside of the office as well as inside.
Cons
The tyranny of distance between it's global office is sometimes a challenge. And all the technology in the work helps, but doesn't solve it. But something not limited to just 99designs with a global footprint I assume.