Pros
Very friendly and flexible place to work. Home working, flexitime, 27.5 days leave, 5 optional extra days leave. Within small teams the environment is generally very good. My line managers have all been really good and there are a lot of really talented engineers in the company which you can work alongside. A good place for a graduate to start for a couple of years but if you are any good you will find a better job elsewhere.
Cons
Performance management is soul destroying, it is designed to make everyone average. It generally bears no relevance to the work you are doing and really destroys your motivation. Bonus and pay are lousy even if you perform well. Senior management generally have no technical knowledge and really are out of their depth. There is a constant churn of strategy ideas and after a year or so you just give up keeping track or caring. It's a two part company, a whole load of senior managers running round not knowing what they are doing, and then a whole load of engineers trying to get work done despite the senior managers. Internal processes to get anything done are overly complicated and really just stop you from even trying. The most simple of tasks involve long streams of totally pointless paper work and duplication of data. This comes from having too many people who are just trying to keep themselves busy but really should not be there at all. A lot of the contracts setup by BT with external suppliers provide substandard services and no clauses to protect BT. BT continually ruins it's own future by failing to check details with its supplier contracts.