Of the airlines I have flown (American, United, Northwest, Continental, and Southwest), the most overrated is clearly Continental. While they were no a bad airline, they were certainly nothing special.
On my first flight, SAN-IAH, I was upgraded to First class, and it was sadly disappointing. While the breakfast was delicious and the service attentive during the meal, the service stopped completely after breakfast dishes were cleared. For the last hour and a half of the flight, the flight attendants came through the cabin exactly once, and that was right before landing to make sure that all carry-ons were stowed. When I'm flying in the main cabin, I have absolutely no problem with walking to the galley to ask for a Coke. In First Class, however, I consider the need to do so totally unacceptable... the service should be attentive enough for the FAs to notice that your glass is empty and offer to refill it promptly. Some have said that such a cessation of service simply caters to business travelers who don't want to be disturbed, but I think this is hogwash. First of all, even when people are working they may want a drink. Secondly, the mark of a truly great airline product is that it is personalized; thus, the flight attendants should observe passengers' activities and customize the service to fit. As such, they woudl be able to offer servce to those who are reading magazines without disturbing those who are conducting business.
My second flight, IAH-IND, was just normal. The seats were comfortable, but nothing to write home about. Despite the fact that the airline's website listed this as a lunch flight, despite the fact that the flight attendant implied that lunch would be served, and despite the fact that American offers a meal on their DFW-IND flight at the same time of day, there was no meal service. The flight is 2 hours and 15 minutes, but there was only one beverage service.
Aside from their "excellent" in-flight service, CO was a joy to deal with over the phone. The ticket I purchased from CO featured NW codeshare flights for the first half of my trip, LAF-DTW-SAN. However, when there was a schedule change, most of the folks at CO were about unhelpful as could be. I spent a total of about 10 hours on the phone with CO trying to get them to fix my itinerary, and, once they did, they made me go to Indianapolis (1.5 hours and $33 away) to have a Continental agent endorse my ticket over to NW (CO doesn't fly to LAF). Upon arrival at the CO counter at IND, the agent said something to the effect of, "Well, this was a waste of your time... this is already a NW ticket. I don't need to do anything to it." I actually spoke to one reservations agent at Continental who didn't know what a codeshare was, and I had to explain the concept to her.
Maybe it's just me, but these don't seem like service attributes of what is supposed to be the premier airline of the United States.
On a side note, I would say that the most under-rated of these airlines is Northwest, whose service I found to be friendly and comfortable on my LAF-DTW-SAN trip.