It's mind numbingly boring to sit through it all again, but it helps a lot if the professor is better the second time around. I'm retaking a course now 'cause I never finished up my INC the first time and I'm losing it 'cause it's the same really bad professor it was the first go around. A weird benefit of barely ever going to class the first go around is that the information is kind of new.
I've had to retake far more classes than I want to admit to 'cause of INCs (and I withdrew from one class), so I'm not listing them.