So has anyone played Harley's revenge yet?
First, yes you should finish the story before playing it. It doesn't give away everything but... it does spoil a couple surprises.
Also, I haven't tried the main game since, but exploration is very limited in Revenge. There are hidden objects to destroy for trophyness. It's VERY short, even a casual player can finish it in one sitting.
Spoilers of Harley's Revenge below, and slight spoiler of an easter egg in the main game:
It's a good ride while it lasts, but well, I don't think it's worth the price they ask for. Recommended if you can get a good trade for the original in exchange for the GOTY edition, because with that version you also get a digital copy of Batman: Year One; or if you're really huge fan of either Robin or Harley Quinn. Otherwise, cross your fingers that someone puts a Let's Play of it on YouTube.