Pros
Plenty of opportunities for progression or sideways moves into more interesting work. The work can have a customer service focus or more back of house admin focus. It's a Unionised environment so terms and conditions are good eg flexi time, and it also means there is support available if you need something eg reasonable adjustments. If you enjoy working with people it can be very rewarding. Depending on the office, it can be a very good team working environment.
Cons
I left for ethical reasons as I didn't like the way the government were dealing with benefit claimants in general and those people that are sick and disabled in particular. So working to implement policies you don't agree with can be an issue. Increasing privatisation has meant that you can spend your day dealing with various different suppliers/providers or dealing with the mistakes they've made which can be frustrating and is often inefficient. Managers are not HR trained but have to undertake HR procedures which can result in some inept/bad decision making. Not enough actual support for staff when dealing with desperate people. Pay is subject to the whim of government so you never know if there will be a pay rise or not. Progression is negotiated nationally as part of the pay bargaining-which can mean you are stuck on the same pay for years.