I tried care.com. Some people who recommend it are actually employed by it and so are recommending it in hopes to increase their chances of employment (as opposed to being moms who use the service).
I tried the service and ended up not using it, but I did talk to several women that were potentially qualified - they just didn't get back to me as quickly as I needed them. (I was also looking for someone ASAP, and none of the ones I deemed qualified were in a huge hurry to start, which is fine, but wasn't fine for me at the time.)
The fee is much less than Sittercity.com's fee... it's $40 for a one-month subscription, so it may be worth trying if you don't find someone directly through another means. I am not re-subscribing after my month is up, though, because it was too tedious a process to try to set up interviews and all that...
Best of luck to you; I know it's difficult to find someone who is qualified and whom you like and trust!