Pros
They offer a preety good salary For some clients you can work with nice technologies. Orion management does not affect your day by day activity ( What I cannot say about the client one) On the client's side, I found developers with which I could work with pleasure. Developers are open-minded and eager to help you if needed.
Cons
Cons are only on the client's side. At least I felt like I was working with management from waterfall times. The layer of architects decides everything, and you can bring new things only if you fight them. But simple developers are great, people with which you can discuss even after you leave the company. Developers from the client side are highly skilled, and they don't suffer from a big ego like Management/Architects (not all of them)