Pros
Interesting challenges to apply technology to innovate in the "local" space, we're way more than a "daily deal" company. My colleagues at Groupon are smart, driven, and fun. We're an "agile shop" organized into small feature teams all doing focused work where we are expected to not just deliver software but design and innovate the product. We've just scratched the surface with the systems and products we'll build to achieve our goals and that means lots of opportunities to learn new things and work on different projects, there are new teams forming all the time. Things are getting better and better for engineers, from an employee driven "open transfer" policy (if you want to move teams, find a fit and go for it), per engineer conference/training budget, and regular hack-a-thons everyone is putting effort into making Groupon a great place to be an engineer.
Cons
It's a young company that grew really fast. Lots of things to get better at like making decisions and communicating throughout the organization. With lots of small teams working independently it's tough if you need to coordinate things. It's been a rough year with poor public press and CEO change out.