not very long and pretty easy. don't get me wrong, i had fun with it but would probably have been disappointed if i paid $10 compared to getting it with my GOTY. if you can get it for a discount (i heard they were doing discounts at one point) then yeah it should be cool.
Story and thematically wise it felt like a pretty good epilogue to what happened in the core game. Addressed a few plot points but pretty much just left you hanging at the end and answered basically nothing.
Playing as Robin was a blast - refreshing change from the norm and added a nice "edge" to what was happening.
But yeah its short so yeah I would only buy it if you got more than just a passing interest in the game.