Pros
Very progressive in terms of flexible working options including working from home. Large number of business areas provide, at least in theory, a wide variety of career development options without changing employer. IBM is a recognised company name throughout the industry which can help in your professional work (in terms of opening doors, getting appointments etc) and also is a name people of heard of outside the industry.
Cons
IBM no longer seems to treat its people well, at least in the UK. Employee benefits have been incrementally reduced with every turn and annual 'update', leading to a very cynical workforce (in terms of 'the company' and its anticipated next steps) which does nothing for morale or productivity. Many long-serving IBM employees continue to be motivated by their pride in doing a good job (for the company, their colleagues, themselves) and wanting to support their colleagues, however overall satisfaction with IBM management is undoubtedly at an all time low.