Pros
I like the flexibility of being a contractor with them. They have great contracts in the Redmond area. I know that if I wasn't comfortable or happy in my placement, they would have worked with me to find a different one (although I haven't had to test them on this). Very accommodating regarding any ergonomic issues. The benefits (if you need them) are good. They offer a decent match for their 401K plan. The staffing specialists are extremely friendly. They seem to really care about placing you in a position where you will succeed and be happy. They are very skilled negotiators - this can work in your favor (it helps in getting you the contract position with the contracting company).
Cons
Regarding compensation, they use a rate scale for contractors - they say that it's based on experience. To an extent it's true, so if you're new to the industry, you may have to bid lower. They take a very high percent of compensation though, so make sure that you negotiate a good rate for yourself - the money is usually there. Seem to have a good benefit program and offer a Flexible Spending Account for healthcare if you are otherwise insured. I'm not sure if they will negotiate to forgo any benefits in exchange for a higher compensation rate, but this would work out to a better deal if you don't need the benefits. I know other contract companies allow this.