If you live in or near LAX, SFO, MIA or NYC you're in luck, as the vast majority of courier flights go from there and you almost have to be in those areas to go last-minute.

Used to be that you almost always earned miles, but more companies in recent years have yanked them away. Definitely check first.

I've flown courier 3 times, all to Asia on conventional, fixed length vacations. It's been awhile, though - last one was in 1994.

Comparing now to then I find the couriers offer about the same or even slightly worse deal than 8 years ago, while other deals (paid tickets or mile awards) have slowly but steadily improved.