IT recruitment and staffing firms both hurt and help the industry, IMO. They hurt in that those that are the incompetent twits that don't get various technologies will go ahead and try to find insane things like, "5 years experience with Visual Studio 2008," which should seem non-sensical to most. They help in that those that do understand technology and how some companies want to have their IT department set up through some contractors, it works well for most involved.
On a personal level, I find that dealing with more than a few agencies can be tricky if not outright frustrating as the same job will appear on mulitple sites and you have to know which company sent your resume for that position as if someone else also applies you, it can take you out of consideration. Thus, one has to have a spreadsheet and usually there isn't any feedback which just makes the whole cycle annoying.
There is a bit of a paradox to these firms in some cases if you think about how they would maximize their profit in placing permanent employees. Those placed have to stay with the company past a probationary period, but then after that they should move on so that the position can be filled again. This is where in a way that want someone good but also someone that will move on soon.
There is also the potential "meat market" feel that some recruiters give me that tends to be something that makes me not really want to work with them. These are the cases where they are trying to fill out your stats, e.g. x experience in a or y experience in b, and then fire your resume everywhere and see what sticks. No thanks, I am already trying that and don't need others to do that for me.
Other places can be interesting to revisit them from time to time and do a kind of passive job hunt, where if something great comes along I may take it, but otherwise I'm happy where I am.