Perfect dark zero was a launch game for the Xbox360 and so it shared the responsibility with other launch games, to showcase the power of microsoft's new console. When it first came out, PDZ was loved by every non Sony or Nintendo fanboy for it's "incredible" graphics and it's "coolness."
That was until the people who had the game completed it and realized that that was it. There was nothing else to do bar play it over Live. There was no incentive to play against crappy A.I. on severely repetitive levels. So, Xbox Live. This is the point, while playing against some poxy little kid who can't shoot straight and shouts down the mic like a howler monkey, that all players come to the realisation that this game actually isn't really very good at all. Behind the now not-so-incredible graphics was a hollow game.
It can be played once or twice, but then the experience is all over, the disc is back in it's box and the box is back in your local game store. And the worst thing is, you realize that you could have just waited for Ghost Recon 3 all along.