I've bought one international business class ticket. It was a 'round the world' flight, and expensive (like $7000), but I had about 30 flight segments. I did a lot of flying in 6 weeks, and to me, even at the high cost, it was worth it to not be dead all the time I was traveling. This was for pleasure.

Otherwise, my business class trips come from using miles. Recently I took a business class trip to Asia with miles I got from getting my latest mortgage and realator through Lending Tree.

Domestic, I almost always get first class, paid for by me, domestic or business (I pay for my own upgrades, if I don't get a complimentary upgrade or an upgrade through credits). I fly AirTran, so I typically buy a coach ticket and upgrade 24 hours in advance (unless a business class ticket isn't much more than the cheapest coach class).

I'm a big person, and I'm more comfortable in first/business, so scale my number of flights accordingly so I can afford this. I'd hate to encroach on someone, especially for a long flight.