"I promise I would help you with housework after this stage, dear!", and 10 hours just whizzed by.
Batman: AA features a lot of puzzles that teases you with I-know-it-contains-something-but-I-can't-touch-it-yet visual hints and entices you to quickly double-back to unlock them once you've gotten that gadget, much like Darksiders. The gadgets' uses on puzzles and enemies are ingenious which coincides well with the Batman universe of physical and cyber obstacles.
I find that Arkham Asylum features a much more balanced mix of stealth, acrobatic puzzles and chaotic combat, which is something like Splinter Cell + Tomb Raider + God of War in a nice 8-10 hour package, which you could stretch double depending on your motivation to solve the Riddler's puzzles, spread around the entire map. The "Detective Mode" adds on more oomph to the already impressive visual treatment of the game, and you would be compelled to toggle frequently between the two in every area just to see how it looks like in both modes, which is an innovative touch.
One of the best cinematic and gameplay moments of the game goes to the Scarecrow scenes and the resulting "boss game", that gives it an amazing twist that the 2007 Batman Begins somehow fails to grasp for the depiction of this villain. On top of all the good words, sound design is the cream of the corp, which serves as the catalyst that immerses me into the game fully. Two thumbs up!
KingLim