Pros
Great experience, a lot of learning opportunities, top class customers, private medical insurance, car for higher bands, global teams and excellent reputation among employers so you can easily get bank loans and grants and a good pension scheme.
Cons
You have to be self motivated to thrive so you own your career within the company, managers focus on sales and numbers beyond anything which is understandable but it might not suit every one especially people from purely technical backgrounds. The market is changing rapidly so IBM is continuously transforming to respond to those changes, so your very hot job role six years ago is possibly worthless nowadays; add that to the fact that the company pays the minimum legal redundancy packages, after 10 years of service you will end up with a £4,790 redundancy package if you did not negotiate a special redundancy package before joining. News about IBM layoffs have been in the news everywhere, but it applies to all tech workers that you should always take care of your career to make yourself of value, this highly applies here as well.