I've been trying to make some headway in the Challenge Maps, which are actually pretty damn hard...especially if you're trying to do trophy duty as well. It took me forever to get the hang of the enemy placement and timing to do the total Silent Takedown for the second Invisible Predator scenario, but it was amusing to observe the guards' reactions as I picked them off one by one without making any noise. In fact, I'd say the Invisible Predator maps are well worth investing time in the Challenge maps, as they don't go for that trap of making you race against a clock (unless you care about online leaderboards, and I don't). Instead, each map simply charges you with taking out the enemies with cetain attacks, which really lets you appreciate the map designs and the tools at your disposal.
I do not care for the combat Challenge maps at all, as I never could get above a 27-hit combo (my personal best in the main story) and you pretty much have to be flawless to get all 3 medals for those scenarios (I've done this once for a trophy). Sadly, I experimented a bit with the much-vaunted Joker, and honestly it's pretty lame. You're just going through the same maps as Batman, but with Arkham guards and hero characters as your punching bags instead of Joker goons. Joker himself controls a little differently from Batman, enough that I have some trouble using him. The Joker's much weaker than Batman and attacks in wierd arcs, but he has faster attacks so he can build up combos quickly. His finishers are really hard to pull off in my experience, so it's harder to chain them together. His moves are really amusing, though (I love the joy-buzzer finisher). I haven't tried him out in Invisible Predator mode yet.
In case you're curious, the Crime Alley map is just another Combat Challenge Map (with medals, oddly enough), and Dem Bones is pretty lame as a survival mode where you see how long you can go attacking the Scarecrow's skeletons without taking a hit.