I got mixed feelings about endurance races. Together with the rallies, they were my favorite in GT3.
In GT4, I find two problems with them, not unsurmountable but still annoying:
1) It is hard to figure out which car in your garage will give you a good challenge while still being winnable. It is annoying to have to try multiple cars, starting a race, running several laps, aborting and changing car... just to try again, and again, and again until you find the one that is worth doing.
2) Many of the races are way too long... more than I'm willing to dedicate to a game - any game - in one sitting. It would be better if you could save mid-race. Leaving the PS/2 paused for a few days until I'm free to continue is not an option for me... besides wasting electricity, my power company sucks and the PS/2 gets reset when the power dips or fails (a common event).
So, the first time around I played most of the endurance races in B-Spec mode. This time I'm trying to play them A-Spec, but I'm being careful about when I start one. Still, I plan to B-Spec all of the races that last more than 3 hours.
Maybe in GT5 we'll have a game option to limit the maximum length of endurance races (wishful thinking, I know).