We used Insperity/Administaff, and our healthcare rates went down by 30% and the coverage got dramatically better. Plus, they added all these adjunct benefits that big companies typically offer that few people use, but don't hurt to offer your team - free legal advice, adoption assistance, cheap LTD and life insurance (and a free life insurance policy for everyone), and so on.
They were organizationally harder to deal with than our previous payroll provider, but it was one of the better backoffice decisions we made period.