Pros
Same reasons you'd work with any staffing firm; Recruiter was encouraging while I was interviewing, but little did I know (see below). As with everywhere it's who you work with, and not everyone there was bad or a liar. The office staff, especially, is great, as was the account manager I saw the most.
Kontras
I was new to field and naive, but recruiter lied about salary range and I was dramatically underpaid and recruiter rescinded full referral fee; Sr. mgmt. yelled at me(!) when I declined to take a vacation time for training they'd previously agreed to pay for (It's standard for the staffing firm to pay for your time and the place you're contracted at to pay the training fee); For an IT staffing firm their internal IT systems are slow and unresponsive, making time reporting (or doing anything in their system) an agony (small factor but still...).