Having tried just about every saddle on the market ("Easy Seat," comfort saddles, Specialized Body-Geometry, Brooks B-17, Brooks B-69, etc.) I still haven't found one that is comfortable for more than about 1.5 hours at a time. I think I've finally figured out why:
A typical bicycle racer weighs about 150 pounds. A typical bicycle seat has only a few square inches of seat area, of which only the three square inches under the sit-bones are pressured to any extent. With a 150 pound racer, the three square inches of saddle being used are seeing a pressure of 50 pounds per square inch (the 150 pound total weight divided by the three square inches of sit area).
If we assume that 50 pounds per square inch is close to the maximum comfortable pressure for longer periods of riding, consider what happens when a 250 pound person sits on the same bicycle seat: The "loaded" part of the seat (and of the behind) are now seeing more than 83 pounds per square inch (the 250 pound total weight divided by the three square inches of sit area). This is 166% of the maximum comfortable pressure!
Note that these weights per area are static weights and do NOT take into account the acceleration forces that happen when the bike hits bumps or potholes.
What this means is that no matter what saddle you get - the more you weigh, the sooner you'll become sore.
In fact, the maximum time for comfortable riding can probably be quantified as well:
150 pound rider X max comfortable ride time = 250 pound rider X 1.5 hours (personal estimate)
Using this formula, even a 150 pound rider would get saddle sore after about 2.5 hours of continuous riding. There may be some acclimitization and other "booger factors" to take into account, but it seems roughly right.
How does this estimate correlate with your experience, and what do you think of this theory?