Pros
I'm having a hard time here. They are a large company (backed by a HUGE company) with lots of resources and access to / contracts with some of the areas larger clients. This comes with it's own set of problems, though, which they seem less than adept at handling. If you lower your expectations (very low), they do provide some sense of stability, but prepare to be micro-managed while your wages stagnate.
Cons
Experis seems less like a company providing services, and more of a legal construct designed as an abstraction of its parent company (Manpower) to expand their traditional labor services into the IT vertical. I say this having worked with the company's previous incarnation, prior to it being acquired by a global corporation. The problem is that they treat technical resources the same way they treat day laborers: as "spare parts" (at best) or as "rent-a-slaves" (at the worst). Maybe their operations on the staff augmentation side of the business are better, but their managed services are (in general) underpaid, under-trained, understaffed, and unsupported.