The greatest Batman game ever made.
Arkham Asylum is based on a graphic novel of the same name, but if you've never read it, don't worry - the game itself has an excellent plot that draws you in quite well as you play. If there is one word that I can muster that best sums up the experience of Arkham Asylum's gameplay, it is immersion. First and foremost is the environment itself. Arkham Island is perhaps the most infamous of all of Gotham City's locales, and its design is exactly what one would expect it to be - dark, gloomy, gothic, and riddled with twists and turns. Whether creeping over the rooftops of the old asylum buildings or making your way through the island's dark caves, the feel of the place just makes you believe that it's real.
The other piece to this, of course, is Batman himself. The experience is perfectly done, from the classic design of the character to the spot-on controls. Batman comes with a set of killer moves and gadgets from the get-go; as soon as you direct him through his first fistfight, you'll be hooked. The combat is 100% Batman and completely entertaining; leading him successfully through a brutal melee against a gang of thugs can be accomplished by following the visual clues that the game provides for you. The fights can be difficult, but are not so unforgiving that you'll feel punished for making a few mistakes. They definitely require a careful and observant approach as opposed to mere button-mashing, particularly later in the game as the enemies get tougher. Thugs armed with specific weapons, such as knives and shock sticks, must be dealt with more carefully than the run-of-the-mill goons - particularly the ones armed with guns.
Luckily, Batman has an entire arsenal of new weapons and combat moves that you can purchase as you gain experience and unlock equipment by progressing through the story. Many of these items are best used prior to a fight, causing you to spend some time observing foes and planning out your attack before ambushing them. This is where Batman's stealth and mobility come into play, as well as the proper use of the grappling gun and cape glide to get to high ground and a good vantage point.
Many of the game's 'set-piece' battles involve waiting in ambush in a closed room with multiple thugs armed with firearms; the only way to proceed (and survive) is to use patience, cunning, and Batman's acrobatic skills to isolate and disable the foes one at a time. While these encounters are tricky, rewarding patience and punishing haste, hearing the increasingly-frightened criminals react to your unseen predations makes stealthily taking down each gunman that much more satisfying. Whether you're engaging in combat or simply grapple-gliding from one point to the next, every moment of the game makes you feel as though you are genuinely controlling the Dark Knight's every move.
While the Joker is of course the main threat, many other villains from Batman's Rogues Gallery appear as well. The voice acting is superb, as most of the talented performers from the animated Batman series were brought in to reprise their roles. The game does an excellent job of pulling the side villains into the main story, utilizing them to emphasize, rather than detract, from the atmosphere of uncontrolled chaos that the Joker has unleashed upon the island.
In true Batman fashion, most of these villains are not truly defeated until later in the game, after multiple encounters. Multiple encounters with the Scarecrow (and his fear gas) are both chilling and unsettling, while the Riddler provides the game's main collection sidequest, challenging you to an isle-wide scavenger hunt involving trophies and noteworthy locations throughout the Asylum. While collecting all of the various riddles and trophies can be a tedious chore (forcing you to backtrack all over the island after unlocking new pieces of equipment) there are also many entertaining easter eggs to be found that veteran Batman fans will get a kick out of.
In all honesty, there is really not enough that can be said about how good of a job that this game does of cementing that sense of immersion and realism. It simply feels the way that a Batman game should feel, and it does it in the same way that a good Batman comic book does it - by making all of the necessary elements come together. There is just that feeling that it is the way it's supposed to be, and Arkham Asylum has that in spades. The only difference is that this time, rather than just reading the comic book, you actually get to play it instead.