Played it yesterday... wow.
Played Perfect Dark Zero, very interesting. If you expecting this game to be a 'Halo killer', sorry no dice. But As far as shooters go, this game was very involving. Played co-op player mode, and there were moments in game that you needed to use the old grey matter. In one point, you are separated from your partner, one on the roof-tops, the other on the streets. And you have to cover each other through the level. Wow. The gamed reeked of made-for-xbox (unconvincing environments effects, like subpar skies and shadows) and ported for 360, but after was said and done (or in my case, bith and moan :p), what seemed like 3hrs of play was actually 5 hours.
So my recommendation: If you didn't get the Xbox 360 yet, don't sweat it. Give the developers time to get a better feel for the system (3-6 months) before the truely great games are released. Rumour mill says a new release for Fable and Elder Scrolls are pending release