We advertise a paid listing with them as well, can't remember how much it costs but i'm sure we pay yearly rather than monthly.
We've had a couple of enquiries but not enough, we wont be re-advertising!
On the free listings side, we place free listings everywhere they accept them - if its free its not going to hurt you (unless its on a site completly unlinked with cleaning) and therefore can only increase knowledge of the company / service.
Free listings are definately the way to go, with a few well-placed sponsored ones that are proven to work for your company (Thomson and Yellow pages, not yell.com for us).