Pros
Malcolm Pirnie was a great company that had an almost family-like atmosphere. Most everyone seemed to be very client focused and proud to work there. As I understand, depending on who your boss is and what the Pirnie/ARCADIS supervisor ratio is, it can still be a good place to work. Good benefits, good compensation, great projects to work on. You don't have to be senior to work on large, impresssive projects.
Cons
When ARCADIS took over, there was a clear divide of people working for former Pirnie bosses and ARCADIS bosses; if you were allowed to take PTO, if you were given feedback on your performance, if your boss cared enough to try to find you work in a different geographic area if your area was slow so you wouldn't end up being fired for not being billable enough - it all depended. Though Senior Leadership was changed all the time though, so it seemed to be a top-down problem - everyone is too concerned about watching their own back. Communication became non-existant. It is 100% bottom-line driven firm now; even Vice Presidents seem to be just cogs in the machine.