Pros
United Healthcare offers a lot of internal mobility, and the chance to take on new and challenging roles. Because of it's rapid growth, and expansion into all types of personal insurance there are a wide variety of roles opening up all the time. In addition, the integration of those products, and attendant software platforms, offers employees the chance shine, and become acquainted with other facets of the business, giving them a greater understanding of the business as a whole.
Cons
Although United offers a great deal of internal mobility, their pay scale seems incomprehensibly rigid. Most of the people I know that have been there for any length of time have left at one time or another and come back. Once you've worked someplace else for a few months, the company seems to have no problem with hiring you back at a 25% premium to your previous salary, but once you're in it's almost inevitably 3-5% regardless of how your skill set has evolved. The other major failing I see is common to most insurance companies, abysmal customer service. Management constantly pays lip service to the idea of good customer service, but I have never seen it as anything more than lip service. Because of the size of the company, there is endless transfering of people from one place to another, and customer service positions seem to be chronically understaffed and/or trained.