I flew as a courier on United from LAX to Bangkok about four years ago. The agency that sold me the ticket normally charged about $400-500 for the roundtrip (compared to $800+ for least-expensive coach at the time). But as it happened, I called during Thailand's low season (to inquire about all destinations they had available), and they mentioned Bangkok for $250, if I could fly in 6 days and stay for 2 weeks.

This is common; if a courier has a shipment leaving on a certain date, they want to be able to charge as much as possible for the seat (to offset their shipping costs). But as the date approaches, if they haven't filled the seat they start dropping the price. I hear that sometimes, if they haven't filled the flight by 24 hours in advance, they will give the seat away for free, just so one of their paid staff doesn't have to make the trip (this deal usually is for repeat customers).

The process was relatively smooth, although one unnerving aspect is that the only time I had the actual ticket in my posession was while on the flight; they met me at the airport for both legs. This is apparently routine, but on the return trip, I was told to be at the airport 2-1/2 hours in advance while the courier rep only arrived an hour before the flight!

As for miles, you can ask the courier agency up front whether you will earn miles for the trip. Sometimes, the deal an agency has with a specific airline does not allow for miles. In my case, I didn't ask ahead of time, and although I provided my UA# at check-in, they didn't post. But I was able to call later and get the miles posted into the account.

From my experience, it's a great way to fly cheaply if you are not too picky about your destination, can travel lightly (you may not be able to check bags), and best of all, can travel at the last minute.