I have dealt with 3 out of 4 of those. I have nothing negative to say about them, other than that, for myself, I'd prefer full-time positions over doing contract work for them. So I can't imagine a situation where I'd leave a full-time position for whatever they're posting, if I were simply job searching while employed full-time. Some of their positions will lead to full-time positions eventually, some won't.
Robert Half seemed like the nicest, but I suspect it depends on the individuals you deal with. Some are nice. Some are sleazy. Treat them as you'd treat any other vendor. Call them back if you need them, don't return their calls if you don't.
I wouldn't expect them to do you any favors. If they place you somewhere it's because their customer picked you, and not because the firm pushed you over other candidates.
I do think, though, that often times the only way to get to the jobs they're posting is through them(and possibly other firms competing to fill the same position for that particular client) because their client doesn't want an FTE or only wants to do contract-to-hire, so dealing with these guys is the quickest/only way to get your foot in the door with that company, or this industry in general.