Pros
Working with extremely talented people Few, if any, dull moments Always pushing for exceptional work Interesting and rewarding projects & clients Constantly learning Excellent support structure for employees, company looks out for the staff (including contractors) Open door policy for mgmt, they are there to help and guide Good culture Almost every project has never been done before People are held to high standards for quality of work, this elevates the end results and ultimately leads to more interesting projects Mgmt listens to requests and acts on them, remedies issues. Did I mention the people? People are allowed to make mistakes, so long as you admit it and don't do it again.
Cons
Project timelines sometimes are not long enough to do the BEST of the best work Company growing out of start up mode includes some "we need to figure this out" in daily work life Almost every project has never been done before (this can hurt sometimes) If this is your first job, or haven't been in the business a while, you may sink unless you speak up