I've already finished the game (100% completion). Loved it.
It's a well built game with no major flaws. The story and characters are a bit bland when compared with GTA games (well, any sandbox will suffer in comparison to those) but good enough to drive the gameplay.
Driving is fun, Honk Kong looks lively and great, and though there is some shooting involved, the shift to melee combat is what sets it aside from other games, I think. The combat is very fun, dynamic and varied. Batman's style of attacks and counters with different types of enemies. Shooting is great and the fact that it's scarce make it for worthwhile moments.
All in all it's a very good game, I think it deserved more attention.