Pros
Strong engineering culture; tech is the core of the company. Lots of nice, smart, hard-working, and fun employees. Relaxed work environment (e.g. lounge, snacks and drinks, ping pong and foosball), top of the line equipment to work with. People have lots of autonomy and freedom to excel. Company is active in the tech community, and pays for engineers to travel to conferences. Good tradition of being introspective, meaning that problems get addressed directly. The business model is strong; there is lots of revenue, and lots of potential for growth.
Cons
Growing pains -- effective organization and internal communication is a hard problem to solve as the number of employees goes up. As a result some parts of the company are disorganized. New initiatives are sometimes poorly planned or executed (either too much or too little up-front planning, or too much change during development).