Sharon -
I seem to recall you and I talking about this topic as we drove from Boston to central CT a couple of years ago!!
There are a number of us, including Pat H, camachinist and me, who include pure mileage runs in our list of enjoyable activities.
Most mileage runners define a pure run as one where the trip is taken for no other purpose than to accrue frequent flyer miles or a higher level of elite status with the airline or both.
As an example, Pat H and I not long ago flew from the East Coast to San Francisco to Tokyo to Singapore, spent about seven hours in the Singapore airport and then turned around and flew back along the same route. We each accrued a total of about 45,000 United Mileage Plus miles (flight miles, bonus miles and credit card purchase miles) on the trip and, with other FF miles earned during the year, earned United's highest mileage-earning elite status (called "1K") for the following year. The total trip took about 60 hours, including the 7-hour Singapore stay.
There are some who earn higher status or more miles by taking an extra trip to see relatives or friends or spend a relaxing weekend, but pure mileage runners don't call those trips mileage runs.
My personal answers to some of your questions:
1. I made mileage runs (all on my own dime) regularly through 2007, but because I am retiring this year, I'm starting to use the millions (yes,
millions) of frequent flyer miles and frequent stay points that I have earned over the years and might never do another one.
2. I was almost always able to upgrade, using upgrade instruments (or occasionally FF miles) earned from earning mega-miles and elite status.
3. Yes, sometimes I stayed overnight in a hotel, but often only if I couldn't get a return flight without doing so.
4. The cost was worth it if the trip contributed to earning the elite frequent flyer status I wanted, especially if the cost of the trip was less than, equal to or only a bit more than the value I put on the earned FF miles (including bonus miles). The best value often comes from flying on mistake fares, extremely flow fares that airlines sometimes mistakenly loan into their fare systems. Pat (camachinist) can speak to that, if he checks in on this thread.
4. (#2) It is worth it if you think it is. No one's opinion matters except yours (and, ahem, your family's.

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5. I have no AA mileage run experience, although Pat H has. My experience is with DL and UA.