ZOE2 is a MUST-PLAY PS2 title, similarly to Silent Hill 2, Shadow of the Colossus, Breath of Fire: Dragon Quarter, Metal Gear Solid 3: Snake Eater, Okami and other PS2 classics. You haven't experienced the platform if you haven't played it.
It's one of the most polished action games I've ever played. Everything about it is smooth, supple, and responsive. The gameplay is excellent. The levels are varied and inventive and constantly introduce compelling challenges. The art direction is superb. The musical score contains some of the best techno/eurotrance in videogames this side of Ridge Racer.
The voice acting is only as retarded as the script, and the voice actors did what they could with it (although it's angering to hear them say something out loud which is clearly not grammatically correct English, knowing full well what they were saying, but doing so anyway). The scenario is interesting, but it's not given a chance to really go anywhere. The opening act is terrific, however, and original enough that the rest of the game can ride its coattails.
What you need to understand about ZOE2 is that it makes playing a mech sweet. It's not a lumbering, slow truck with horrible turn speeds (although at the start of the game you will be piloting one, as if the creators wanted to put you through that to make you appreciate what's to come). This is an Orbital Frame; you're going to be zooming back, dancing around your enemies before beautifully exploding them with amazing attacks. Everything is subtly cel-shaded, including amazing-looking particle effects comparable to Zelda: Wind Waker, which makes gliding through the environment all the more enjoyable. As Mr_Domino said above, the feeling of true 3d control you get in this environment is unparallelled; since you're frequently in wide-open surroundings, great camera is a requirement, and they execute on that requirement brilliantly. You are always aware of the distance between yourself and your enemy, partially through the aforementioned camera and partially through superbly varied enemy designs, which allow you to pick out your foe from among the many mosquito-like packs all around you.
The game's length is "just right" at six hours, and I say that because like other short games (Portal comes to mind) the gimmick of flying around in a mech isn't overdone by splurging average level design all over the game. Each level is a concise representation of a certain aspect of play and there is no need to copy that just to force the player through another drawn-out level. The game has great replayability because of it, as some levels are truly memorable and worth going through several times.
As any great Hideo Kojima-produced title, there are plenty of alternative game modes that, yes, I actually play (which can't be said for most games). The most interesting is clearly - and this may only be available to Europe, since we got a sexed-up release of the game with special features - the ability to instantly load any level you want, as any of several mechs you want (the original Jehuty through Naked Jehuty, which is normally only available at the end of the game), where you can activate certain challenges, such as time trials. There have been many times where I'll just load up the level where you're in space and just beat the shit out of endless streams of enemies, teleporting around and having a blast.
ZOE2 intends to be epic, and the buildup to its well-executed epic climax is continuous and exciting. You'll start off feeling like a badass simply because the game throws hundreds of tiny "mosquitoes" at you, which you can either shoot at (boring) or sprint into and slash away at with your elbow blade. As the game progresses, you'll find yourself facing off against enemies reminiscent of top-down shooters: heavy artillery with slow, avoidable attacks; stationary cannons; zippy, light-weight fighters; enemy Orbital Frames, and even a fleet of battleships.
As Mr_Domino describes, you can expect truly jaw-dropping set pieces. There are only one or two so-so, drab levels which take place early on in the game; the rest is essentially one long dance of death. You'll fight a fleet of battleships, approaching each one and taking out its defensive fighters and long range cannons before landing on its deck and unloading a supercharged blast into its engines. You'll ride the top of a high-speed train, jumping from wagon to wagon and taking out anything that stands in your way. You'll fight thousands of enemy troops on a battlefield where you're the hero, slashing through wave after wave easily while your comrades struggle to survive. You'll fight in space, on Mars, inside high-tech, futuristic ships and plants, in Neo Tokyo-esque cities, within industrial structures. And you'll learn an ability so incredibly awesome it changes how you played the whole game up until then.
Finally, Zone of the Enders: The 2nd Runner is one of the best action games ever made. If you consider yourself someone who partakes in the very best artistic titles, you have to play it. Wow. I've hyped myself up to play it again now.