I'd never thought much of the Supershuttle service in DC (it wasn't bad, but it wasn't great either), but since moving to New York it's become my transportation of choice for going TO the airport.
Pricewise, it's cheaper than taking a cab out of the city (there are flat rates in place and nearly all end up close to $50). My fare as a single passenger is $18-20 with tip.
Timewise, they do pick you up a bit too early, but I'd rather have too much time at the airport than too little and run the risk of missing a flight. And, I admit, I enjoy that extra time at the airport to get checked in and partake in the relative novelties of junk food and trashy reading. I've never missed an international or domestic flight when running on the Supershuttle's schedule (though it sounds like other reviewers have...).
Ridewise, I enjoy the Supershuttle because it takes me through parts of the city that I otherwise don't normally explore -- it's nice to cruise through Harlem and the Upper/Midtown East Side as a passenger. The drivers are usually friendly and in relatively good spirits -- once, on a 4am drive, the driver roused our van full of zombie passengers with a breakfast of disco, and we were all singing along by the time we reached JFK (delirium could have been part of it, but honestly, it made the drive fly by).
In seven years of taking the Supershuttle (three years in NYC), I have only had two experiences that I can count as bad ones. Once, the Supershuttle called me an hour before they were due to pick me up and told me that there was no van coming -- I was able to call a car service and make it in time, no problem, but the cost was considerably higher. The other time, the driver got bogged in traffic around the Thanksgiving Day parade (what a nightmare) and we got stuck in Midtown. We hit a blocked street (trucks) and the snotty passenger who was our last pickup refused to walk to the corner to meet the van, insisting on being picked up at the door (which was impossible). It was pouring down rain, traffic was at a standstill, and honestly, I can't fault the driver too badly. He told us to all get out and try to get cabs, which we did. I still made my flight, though most of the other passengers missed theirs. At any rate, my good experiences far outweigh these two unfortunate incidents.
As far as getting from the airports back into the city, I rarely take the Supershuttle -- it just plain takes too long (plan on several hours). Also, at the end of my journey I usually just want to get home ASAP, and so I'll shell out for a cab.
But I do recommend taking the Supershuttle to the airports, if you don't mind going early and having a bit of fun along the way.