Pros
There's reasonable job security, and I worked with some decent people. You get free broadband. There is the possibility to learn a lot about telecommunications if you are in the right role. I had a good work-life balance, though I know others who didn't.
Cons
BT absolutely 'misled' me regarding what the job entailed, meaning I was left doing a job in which I wasn't interested and which didn't offer the development that I had been promised. Managers seemed to treat everyone as a numbered resource first and a person second, so felt they were completely within their rights to do so. Most of these managers seemed like decent people who were just used to the company working that way. I know other ex-BT employees who went through the same experience. I know far too many people off work with stress, or just managing to cope with it. There seem to be several reasons for this including over-work and not being listened to. One colleague was working weekends to keep up with the workload, and was also asked on more than one occasion to do work while on leave. BT is a very bureaucratic organisation with an enormous middle-management layer. It is also a technology company run by non-technical retail managers with little feel for technology, meaning that promising projects just peter out; there was a very large cut in the research budget/ department after one of the commercial arms made a large loss. There is quite a significant "personality culture", meaning that who you know and how much you swagger seem to have more of an impact on progression than your ability to add value for the customer.