The factory one is pretty good. It is carpeted on the top side and rubberized on the bottom. I'm not sure it is completely waterproof, but it appears to be pretty close. I'd probably spray it with scotchguard or equivalent - especially if you got wheat interior color. It also velcros to the cargo area so it doesn't move around.
I'm waiting to see if Weathertech comes out with a version. I suspect they will, and it will probably be a rubber/vinyl version that you can hose off when it gets dirty. In the past I've always bought Weathertech cargo protectors for my SUVs. If you need something immediately, the Infiniti one is the only choice I know of. And it isn't obscenely expensive like factory options sometimes are.