My experience with agencies seems to tally with this; starts off OK busy wise, but peters out. I find the quality of clients not great, either a lot of drug dealers/addicts or pedantic, picky, demanding men who like having a theoretical 'boss' they can whinge to if you didn't do quite enough back flips for them . Sometimes I feel like when you are new they give you the most difficult guys that other workers have stopped wanting to see. As someone else said, I think agencies might be OK if you are just trying to get some money together quickly... but I've seldom come across any long term positive experiences of agencies, expensive ones or otherwise.
Is there some particular reason you want to do agency work rather than indie? If you want to have a 'charge high/low volume' model, I think you'd get a nicer client base if you were indie and had a good marketing strategy.