Pros
If you are looking for a company that is investing in significant growth and addressing a great market opportunity, this is the time to get in on the ground floor. The company is make the financial investment to grow as well as the cultural investment to make it an environment of people looking to grow their careers and are willing to work hard to do so. I've spent a lot of time at other companies as they go through tremendous growth and it a very rewarding experience if you're career oriented and want to be a part of something big.
Cons
There are some growing pains and there are at any company experiencing rapid growth. It's a work hard and be rewarded environment. It's not for everyone and that's okay. If you're career minded it's a great place to be.