I've only recently (last three years) been in a position that requires a fair amount of travel, but in that time I've flown close to 200,000 miles with Delta, about 80,000 with American, and around 20,000 miles with United. I've also flown the odd flight on B6, Cathay, and WN. All of this travel was booked in coach.

In my experience the customer service and in flight experience with Delta is miles ahead of American and United. As Platinum flying out of Boston I've been pleasantly surprised with my upgrade percentage (including, shockingly, making BOS-LAX on a Monday morning and then later that day getting the bump to HNL). But even when I am in coach (more often than not) I found the flight crew to be significantly friendlier and courteous.

American and United have both been pretty hit or miss for me. I've had several odd minor things happen with each carrier that just left a bad taste in my mouth. None of these were major issues, but small things that contribute to an overall unfavorable view of the airlines.

My only real regret about investing so much of my travel time in Delta is how hard it has been to get any real value out of Skymiles. I've been banking for an upcoming honeymoon, and even with close to 400,000 Skymiles it's hard to find a business class itinerary that works well.