Pros
There are so many opportunities for learning and advancement within Cicso. It's a fantastic place to continue to hone your technical skills, and what better place to do it then at the company that made the router ubiquitous. Some may say that Cisco has become slow or unresponsive as it's gotten bigger, but I never worked in a company with so many people willing to go above and beyond for you and/or the customer. You won't get as many stock options as you would at a start-up, but Cisco stock is actually worth money.
Cons
It really depends on what your position is in Cisco. In pre-sales where I am, the main downside is the difficulty of getting features that I need inserted into the code so my customer will buy Cisco. Other then that, there really aren't any downsides as long as you work hard.