Limores.net (Now Global Transportation) is by far the most consistent, economical, and convenient service I've found. I use a car service 6-8 times a month and I love their iPad/iPhone app and the fact that I can ^
any new drivers I get right after the ride. I get my preferred drivers around 80% of the time.
I haven't used their Town Car service in a while but their MB S550 service is under $100. (Manhattan to JFK) and you get a discount by booking online. The cars are usually texting me around 10-15 minutes before my given time so I've never had an issue with timing. The drivers are friendly and remember me and where my apt is in the city and the quickest way to get there.