We go here: http://www.thepediatricgroup.net/ and see Dr. Adatto. I think she's great so far (my son is only 8 months old). We were also visited by Dr. Barnhard when my son was born and he was very nice. The office staff is incredibly friendly. We've only been for well baby visits so I'm not sure about short notice appointments, but I've never seen the office overcrowded or waited long for a scheduled appt. They are located in Tarzana, just next to the Tarzana Providence hospital.
We are going to see Dr. Stiles here: http://agourawestvalleypeds.com/index.html
My sister takes her daughter there and says all the doctors in the group there are wonderful. They have an office in West Hills and one in Agoura Hills.