We've had it both ways--terrific and terrible. AK was great, but it went from 5-8, while all the other parks were still open. Epcot and MK we've had good nights and bad nights. MGM was a nightmare--30 minute line (in the rain) just to get the wristbands, then another 30 minute line waiting for the RnRC EMH fastpass line to open up, then an hour and a half wait for our fastpass window to open up.
Even if it's crowded and you don't get a whole lot of rides in, it's still nice to be able to be in the parks after dark, when it's cooler and all the lights are on. My dh swears the parks are more magical after dark!