I've been doing them since about 1990. I flew BOS-ORD monthly for about a year and a half then. Everyone's fares were the same. I could have taken a non-stop with AA or UA for 2x867=1734 miles. Instead, I'd take Delta via Cincinnati for 4 segments at a minimum 1,000 miles each. (The current 500-mile minimum came in later, with SkyMiles.) But it gets better: at that time they gave class-of-service bonuses for where you sat, not what you paid, and Medallion members could upgrade a round trip for 2,500 miles. I'd end up with 5,500 miles (2x4,000 - 2,500) and an F seat for Y fare.

Of course, since no sane person would connect BOS-CVG-ORD when other airlines flew non-stops, DL didn't bother to schedule good connections. As it happens, the return trip to BOS worked out well, but the westbound part involved a three-hour layover. My travel agent (remember them?) arranged with the airport Hilton for me to take their shuttle bus, change at their fitness center, go for a run, shower and take the shuttle back. It was a nice area to run through at the time, with horse farms and small country roads. (It may have changed since.)

My first status run was around 1994. Year-end was approaching and I had 49,000+ DL miles. My TA found the cheapest round trip from any Boston-area airport to anywhere, LGA on a Saturday. The flight attendant (singular, not plural - it was a commuter turboprop) was surprised to see someone who had just gotten off her plane boarding again for the return, but got it when I explained.

Since then I've learned through FT that many airlines allow a little leeway on qualification mileage. If I had known about that at the time, I would have at least called Delta to to check that before booking the trip.