Nanny placement agencies?

My husband and I are expecting a baby in January.  I will be returning to work part-time.  We are looking for a nanny to come to our house part-time on the days when I am working.  It seems like it is easier to find someone full-time vs. part-time.  This makes me think I should use one of those nanny placement services.  I've seen Metropolitan Nannies come up a few times.  Does anyone have any experience or recommendations?

Thanks!