Pros
Excell has to be one of the best contract agencies to work with in the Seattle area. The company provides you with an HR rep, and most of them are very responsive and really get to know your needs as an employee. Their recruiters are also much better (admittedly, your mileage may vary) than other I've worked with in the past - much more able to match your needs and desires to a position that also matches your skillset. Other contract agencies I've worked for seem to try to place you, regardless of fit, simply to make their contingency. They give you an "a la carte" benefit plan and pay based on a sliding scale for the benefits you take. Since their medical/dental benefits are average, I stuck with my wife's plan and it increased my hourly base substantially. Overall, if you're going to contract, this is the agency to do it with.
Cons
They fill a lot of positions at Microsoft. Almost to the exclusion of all others. That's only a downside if you'd rather not work there or if you need, or want, employment during that mandatory MS 100 day break each year. The benefits are pretty average, as agency benefits go - don't know if that's an upside or a downside. Their HR site (viewing payroll, time-off, etc) could use help... it's in, like, 4 different pieces, and you have to log in to each separately - more of a minor annoyance than anything.