I think there are three kinds of mileage runs at different degrees:

1. The kind where you never leave the airport at your destination. For example, flying DFW-LAX on a Saturday evening, and taking a 12:05am red-eye back home. I've never done one of those. I don't enjoy the flying experience that much, especially on a full flight.

2. Taking a trip that you would like to take but really couldn't justify except that you're getting either needed status miles or a boatload of bonus miles. I do about six of these trips a year, and have a list on my Handspring of all the runs that I want to make. I check the prices on these runs every few weeks. I don't worry about ground costs, since there is value to the trip beyond the miles.

3. Spending a day by turning a nice straightforward non-stop into a long multi-stop journey to get miles. Example: ORD-BWI-JFK-BOS on AA gets you 800 more status miles, but it probably adds 4 hours of extra travel time. I don't do these as a rule unless they are much cheaper than the non-stop. I'd rather work during that time and get some flextime to go to Paris on a $399 fare for the weekend (#2 above).