Difficult to master, but when mastered, this feels not suprisingly like Halo without guns. But with some guns...
Yes, the game is starting to get a little old now, but this is one of my all time favorites. Deep story with serious twists here and there, lots of background information about the mysterious super-corporation driven sci-fi world you're in by accessing terminals here and there, and a nicely balanced mix of melee and firearms combat.
Of course, the game's weapons seem almost all like tools for melee combat, such as my favorite weapon, the Van Der Graf pistol that stuns all enemies in front of you for a short amount of time - just enough for you to get in there behind then and do the always wonderful back-breaker move. If you prefer to run around killing everything with weapons, then Oni is probably not for you. You probably could do it, if you wanted, but as you can only carry one weapon at a time and easily lose it if disarmed by an opponent in melee, you must at some point slap some behinds with your fists and feet.
While the graphics suffer some, I admit, with large open and often freakishly empty areas and textures, it's all absurdely pulled up by the attention to detail. Some smoke coming out from a vent, not to mention the ambient sounds from the city and the music, air planes slowly drawing lines in the sky, the smattering sound of a plasma shot hitting a wall. The sound in Oni is superior to many of today's similar games, with good voice acting (a bit overdramatic at times, but still) and lots of "barks" from enemies. It feels like you're fighting annoying people and not just lifeless drones.
There is an interesting cheat mode in Oni that lets you play as -any- of the characters currently on the same level as you. That means that you'll turn into any of those characters and get their attributes and attacks, which gives new life to the game once you've finished it.
Personally, I've played Oni so many times now that its' charm has started to fade some, I admit. I'm starting to see what it lacks more now, such as a multiplayer mode, versus or cooperative. I'd love to see Bungie building upon their Oni-verse soon and give us a game with finely balanced melee and firearm combat, with multiplayer support. Perhaps, since "Oni agents" are mentioned in the world of Halo, they can make some sort of spin-off of Halo where you play one of these agents. Or maybe Oni agents are something totally different, just with the same name...