Pros
Since you're working in the customer's office a lot of benefits are provided by default by customer, such as free meals, gym, parking. The projects might be interesting, though they are short-termed (around a year) and usually are internal to customer. There are external projects, but they are usually in maintenance mode. Lots of possibilities to improve company's internal processes and directors are agreeing with employees initiatives to lead the improvement. Not bad relocation process.
Cons
Lack of leadership: couple of times I heard from managers: "we're going to do whatever we're paid for". Lack of clarity in processes: performance reviews exist, but means nothing in terms of salary raise or promotion. No technical mentoring, resource managers are unable to advice on how to improve as an engineer.