My experiences has also been negative. My personal opinion is that I would advise any administrator to shy away using an agency.
The agencies are primarily paid by placing bodies in classrooms with little regard to experience, qualifications, etc. I've seen this personally, first hand as one of three direct employees at a school needing about a dozen farangs. And notice I said agencies...3 or 4 I think were involved.
Having said that, using an agency also relieves the school of those tiresome little details like providing WP's, checking documents, etc as all of that is expected to be done by the agency. The agency gets paid, a good teacher is screwed, a bad teacher pisses off the school, but at the end of the day, the agency just throws another body on the floor and everyone is happy again.
Now, I'm absolutely sure there are reputable agencies out there. I hope to hear about some positive experience from posters using agencies!