Vantaggi
BT is overall a good company to work for according to my experience. Great training opportunities if you are just starting your career but also if you are not. If you are very proactive and seek training opportunities that relate to your day to day job, very rarely you will be denied the opportunity. Another good reason to work for BT relates to job safety and long term opportunities. Unless you misbehave in terms of work conduct chances are that you will never be fired. If you are looking for a safe job, this is a good employer. On the other hand, being the company big and diverse, it also gives you opportunities to change jobs without having to move out of it. But you need to be very proactive in seeking opportunities.
Svantaggi
It allows you to become complacent with your job. Though this make you feel comfortable and safe, it does damage your career prospect in the long run, especially if you were to be seek employment outside BT. Within the research dept the funding is getting smaller and smaller due to outside pressures. This causes higher and higher competition for promotion and usually creates unsatisfaction.