Pros
First and foremost the people here are great. Work will always be work, but when surrounded by outgoing, hard working, and friendly people, it sure makes work a better overall experience. Besides the people, the product is great. This is an exciting field to be in, and UserTesting is leading the charge all things UX All the "classic tech start up perks" are here so why elaborate. I wear a tee shirt, we are fed good meals at least twice a week, we drink both in and out of the office as a team, fully loaded kitchen, people bring dogs....it's VERY casual in a VERY good way. People work, make no mistake about that, but after the work there's always a good time
Cons
As long as you are a motivated person, and CARE about doing well, there are no cons. If a lack of management and direction is a con in your eyes, then start-ups probably aren't for you anyways.