We used a couple of nannies from Care.com, and they were both great. One moved away after a few months. The other one took care of our kids for 2 years. She was wonderful with the little ones and had a great attitude. Always on time and offered good suggestions on teaching and discplining (she had an education degree and had been around children more than us - then "first time parents").
The nannies on Care.com generally charge a high rate than the nannies that people pay "under the table," but the rates are negotiable and a lot of them hold college degrees and are CPR certified. We only interviewed two people, and they both turned out great.