Pros
Friendly, supportive culture organized into smallish groupings. Informed experts around every corner. Huge Intranet full of information. Modern offices: open plan, reasonable air conditioning and lighting. Ongoing monitoring of Health & Safety: so good for optician tests, display screen setups, etc. Some excellent catering facilities (depends on site) Leading edge telecoms technology. Modern HR approach - if you've just read about some new HR technique, they will already be using it. Not that hierarchical once you get to middle management. Salary is good for the type of job and sector.
Cons
Organization is not as flat as it could be, despite several attempts to flatten it. Technical expertise is in lower management, not upper. Senior management do not always communicate their strategic thinking very well. Detailed performance management planning, recording, monitoring, etc. You agree with your manager what you will do, then you need do it better than you agreed in order to stay still. Lots of mandatory testing at annual, biannual repetitions. Some for regulatory and legal reasons, but lots of repetitive internal testing - like annual virus awareness checks: "What is a virus? Is it [a]...[b]...[c]...?" Slow response to change. But it is a large company. Lots of middle managers who manage lots of other middle managers. Tricky to create change. Lots of management by email or Intranet. Quite a lot of remote management where you never meet your manager face-to-face.