Pros
It's hard to think of twenty words to fill the minimum requirement of 'Pros' section, but the Gherkin (30 St Mary Axe) has good views from the 26th floor. Also, coffee is free, instant coffee anyways. If you have no other offers and/or options, accept an offer by all means, but start formulating your exit plan from day one. A long stretch at ION will count against you in your CV, as most clients whom deal with the company have a negative view of it. A politically savvy person can make a successful career at ION, but if you are not (be honest with yourself now), regardless of how technically competent you are, in the end ION experience will be detrimental to your further prospects.
Cons
A very demoralizing environment. Senior management and sales people treat client concerns with open disdain, and regularly refer to clients in derogatory terms. If you have worked in a Tier 1 organisation the lack of professionalism and complete lack of respect of clients and of colleagues will surprise you, as will the opaqueness of the organisation. Some individual team leaders do try to buck the trend, but that's an exception to the rule. The recent 'LEAN' initiative to change the culture is nothing more than a cosmetic initiative, and that is clear to even the slower members of a very mediocre organisation.