Pros
Allstate is a great place to grow professionally and the corporate structure is such that everyone supports one another. It's not uncommon to see people that have been with the company for 20, 30, even 40+ years. You can easily create a whole career path within the company and have it involve some completely different jobs, because the company is so big. For example, I have a co-worker who started as a financial analyst and went to HR, then to IT, and is now in marketing. Management also thinks it very important to keep their good employees around, so they like to keep them satisfied and happy. And if you work hard, a promotion is undoubtedly around the corner; it will not go unnoticed. Also, Allstate makes a commitment to the community, and you can really see how much Allstaters really care about each other and about other people. They donate books, clothes, money, etc. as well as volunteer their time to better the world one step at a time. Just knowing that the Allstate Helping Hands committees are there gives you a warm fuzzy about working at Allstate. It is definitely a solid company with a good practices and a great reputation.
Cons
The downsides of working at Allstate are probably similar to the downsides of working anywhere else. There are the long hours (depending on which department you work), there is the seemingly unnecessary red tape, there is the occasional incompetent leader, etc. Nothing really sticks out in my mind as being a major downside of working at Allstate. Except maybe that if you tell people you work at Allstate, they will ask you for advice about their insurance/claim, no matter what your actual position is.