Personally, I've been to Metro Infiniti in Monrovia a bunch of times and I've never had a bad experience. Ask for Cheryl if you go there for service.
If you're looking for a small independent in the area, I recommend Arcadia Service Center (40 E. Duarte Rd) for any car basic service. Samy is that rare honest mechanic. He's never seen my Infiniti, but a few other of my cars throughout the years.