Limores.com
I've been using Limores.com for a few months now and have been really happy with them. Not only in NYC but in London and Detroit as well. Things I love about them:
Best prices for Mercedes Benz service
Online chat is very helpful
Online reservation and iPhone/iPad app is super convenient
Follow-up email so you can save the drivers/cars you like and those you don't.
Professional drivers with nice, well-kept vehicles.
Negative
Even though you "pay" online and print a voucher, most of the time you still have to produce a credit card and sign a slip that they run. Not the end of the world but not as convenient as Carmel and Carey.