We've had some good luck with a single one, but only because a) he was really hungry at the time b) he really went out of his way to build up a lot of contacts with a lot of top-notch talent. In a lot of ways, he was a lot more like an executive placement consultant, only placing software developers.

The large bodyshops, OTOH, seem like a colossal waste of time to me. I don't trust them, and the candidates they offer have been almost uniformly disappointing.