I went to Healthy Kids Pediatrics, formerly East Windsor Pediatrics. I loved them there. My younger brother still goes there and I know many families that still use them (I'm 25 and living in North Carolina currently). I would definitely recommend them. They now have two locations, one in Lawrenceville and one on Princeton Highstown Rd in East Windsor.
We use Pediatrics Day and Night in Hamilton. They have regular appointment hours 9-5 and then walk-ins every night from 5-12 and then 12-5 Saturday and Sunday! They were my pediatrician and now are my sons.
Re: anyone have a pediatrician they can recommend in Mercer or Freehold County?
Princeton-Nassau pediatrics has a West Windsor office. We will be using Dr. Riddal. They came highly recommended by several of friends. Best of luck!