My company works with user research recruiters regularly. Here are some insights about agencies before you go ahead and employ one:

1. How may user research recruitments have they done earlier. The more experienced they are, the more they understand that user research recruitments are done on psychographics and not on demographics alone (as in Market Research). If the agency is not experienced, you would have to work a lot to get them to deliver acceptable quality of recruitments.

2. User recruitment agencies can be found across the globe. While working with them in different countries, we have found out that agencies from high trust countries (like Singapore) usually found far better participants with negligible screening. However, in low trust countries (like India), agencies will provide participants of 'passable' quality with a lot screening time at your end.