Pros
BT is a great place to get your foot in the door and begin your career. You will meet lots of great people, learn a lot about what does and doesn't work, and build a great foundation for your working life. BT does a little bit of everything. Working practices are usually pretty flexible providing you don't completely take advantage. Lots of people work from home, the location for most roles is entirely flexible, you can take time off for medical appointments etc. very easily and there are no core contractual hours. Just be contactable at reasonable times of day and there is usually no problem. Free broadband and certain discounts on other BT products.
Cons
Despite being a good starting point BT has a known and wide spread reputation for poor progression, low salaries and paying less than the competition for the exact same role. Even executive and Board compensation lags behind the rest of the FTSE 100. On the subject of progression, you can forget it entirely if you have a technical role. BT only promotes non-technical people, or technical people who move into non-technical roles. This results in talented technical staff becoming full-time managers and "downing tools" leaving lesser skilled people in their wake. If you're on one of the talent programmes your entire success depends on where you start and mostly comes down to luck. Moving around the company to do different things is not as easy as HR would have you believe. Promotion pretty much just doesn't happen unless you are based in London and essentially groomed from the start. BT speaks very highly of its young graduate and apprentice workforce, but the low pay and lack of progression means that even the most talented of these people often move on very quickly. There is no retention of talented people and BT is often just used to get credentials on the CV. Work-life balance isn't good. Too many people are working like trojans well out of hours or "working late" at home, this is considered normal. I often came in on Monday to find e-mails sent on Saturday night at 11pm, or on Sunday lunchtime and people were aghast that I hadn't already seen it. The whole company is bogged down by unnecessarily elaborate processes that mean things often just can't and don't get done in a timely fashion. Everyone is fighting for dear life just to get the basics achieved and this results in constant aggressive escalations, something which creates significant friction and discord. Life at BT revolves almost entirely around performance management cycles. People are just doing things for the sake of their rating (and possible bonus) and not because it's worthwhile or useful, there is no reward in BT for simply getting on with your job. It's a demoralising "two-way" system with a "conversation" that amounts to being told how bad you and that's that, case closed, no defence or plea of mitigation is accepted.