Pros
Excellent vision - the product ideas are wonderful and will really transform the space. The founders are at the forefront of creating the vision. They are very customer-focused. The market for this product is HUGE, and the vision is to make it so usable that every business user can easily use it to manage their business on their mobile device. Employees are smart and helpful. Hoarding knowledge is not tolerated. Politics is minimal. I have worked at big companies and start-ups before - they are the most professional of the start-ups and still have the small-company feel (you can do many things and are not pigeon-holed into working on a particular feature). They are growing. As an employee, there is plenty of opportunity to advance or change career directions - many people (including me) have changed departments here more than once. They work hard to keep the corporate culture which emphasizes integrity and customer focus, and make the office a fun place to work and be productive.
Cons
As a start-up, there are growing pains. Processes are not as smooth as they could be, and org changes are frequent.