Pros
BT was a great place for Research until recently. It's still fairly good (there are opportunities for EU & UK collaboration, papers, patents, membership of professional organisations etc.) but focus is now very much on short-term value to the operational parts of the business (generally as defined on their terms, not ours). Research has lost 50% of its people over the last 2 years, & this (painful) process was managed reasonably well, suggesting that management do care about how they treat people, & there were no (official) compulsory redundancies. Overall package is good for people such as me who have been here a while, although it seems much harder now for graduates to reach the higher levels. Final salary scheme was closed to new members several years ago. East Anglia's a really nice place to live.
Cons
Many others have commented on Performance Management, & it sucks. I've no idea whether there's a hidden agenda to force people out, but the approach seems to be to focus on perceived weakness & to treat strengths as merely 'business as usual'. There's lots of management-speak about 'raising the bar', and the performance 'deal', but from where I sit. it seems that a quota of people seem to need to be seen to be failing (management deny this, of course). It alienates people & leaves them little option but to compete with their peers (which isn't what's needed in a research organisation, in my opinion). Line managers may not like implementing this, but they have no choice. As someone who neither wants nor is likely to get further promotion, it's not so bad, but I can't imagine many younger graduates staying for long unless they are generic, suited, powerpointed high-flyers, or value the location & stability over career progression.