Without wanting to sound too snooty, I found over many, many trips that the discounted car services are perfect examples of getting what you pay for. Carmel, Tel Aviv, Allstate, and the like are known for less-than-stellar service. Having a reservation won't assure that a car will pick you up, coming or going to the airport. Many of the cars are dilapidated, dirty, and downright scary looking. In addition, you'll find that many of these drivers don't know the city well. It's bad enough when they're not familiar with the many shortcuts around expressway traffic, but when they don't know that Manhattan avenues run up/downtown and streets run crosstown you know you're in trouble (and I'm not making that up!)...
Recently I picked up a Carmel car at JFK when my flight had arrived hours late and I was unable to notify my regular service. The driver solicited me for a fare and because the cab line was at least 100 people long (something else to consider if you want to cab to Manhattan is the day and time of arrival - cab lines can last an hour or more) I decided to give the guy a shot. The Town Car he led me to was early '80s vintage, though he insisted it was a '97. I told the guy I might be stupid for paying him $80 to ride in his car, but I wasn't so stupid I didn't know a 20 year old car when I saw one. In the end he had a good sense of humor, but we both knew he was taking advantage of a bad situation for arriving passengers.
At any rate, I've learned that if you want a truly dependable and professional car service, it's going to cost about $80 from JFK, $55 from LGA, and $90 from EWR, including tip and toll. Not cab prices to be sure, but well worth it for me after a long week away and a long flight home. For the occasional traveler it might be worth it to hope for the best with the discount services, but I need to know the car is going to show up, be on time, be clean (and preferably less than 10 years old!), and most importantly, that the driver knows where he's going.
Sorry if I've rambled, but around my office the the war stories about car services far outnumber the ones about airlines!