Over the years I have used staffing services, but for the most part I do everything I can to not have to use them. In the long run, I think it is more of a headache.
Some of the good things - depending on the agreement/contract you sign, most of staffing services I have worked with will do all the initial paperwork/screening for you (interview them, drug test, background check, tax forms, I-9, etc.). Depending on the nature of the job you are trying to fill will depend on if you meet them before hand to interview them or just have them show up on the first day you need them to work. The staffing service is paying the temp employees so you just get a bill from them for the service. If you don't think they are doing a good job you just tell the staffing service and they will find someone else for you. If you really like the person, you can work out a deal to put them through your normal hiring process.
Bad things - in most of the cases I have rarely found someone that I wanted to keep long term in the job or make a full time employee for my company. Don't get me wrong - there are some good people that work as temps, but for the most part I have had more trouble with these temp employees then good experiences. Also it can be a lot more expensive, in the long run, to put a temp worker in a position then to go out and find and hire a permanent employee. Temp labor is not cheap.