Pros
Sq1 is producing some of the best results in terms of sales and optimization available today. Sq1 takes the latest technology and apply these tools on behalf of our clients. I know, because I am involved with monitoring these results and reporting on them. I see where our client's performance was before Sq1 took it over and where we are now.
Sq1 has never lost a client and while I've been here I think maybe 3 or 4 employees left on their own - most of the negative reviews here were written by someone who was let go. Yes, Sq1 is not for everyone. If you are responsible, accountable, very intelligent and mature, you will do well. If you want someone to baby you - then don't even interview - you will be ejected by your peers.
What do I mean? Everyone relies on everyone else to get their job done for our clients. We have UX design people, Optimization People, Digital Media People, Digital Creative People, Account Service People, etc - all whom rely on one another.
We are creating something special.
Cons
Fast growth requires lots of hard work - being tied to performance metrics is sometimes stressful. That is what I like however.