I have the factory cover (NOAH material) and use it every day. It works great, just don't use it in high winds or it will flap against the paint. If going aftermarket, you might consider the Weathershield fabric. It is much thinner, lighter and folds more compact, but provides great protection (the factory Z and G covers are of this fabric). The advantages of the NOAH are that it is a bit heavier so as to be able to stay put in light to moderate wind and it has a bit of thickness to it that may provide a small amount of cusioning against impact (hail, etc.). The cover keeps my FX very clean and repels rain fairly well. It will soak in heavy rain, but channels the water down the sides so your paint won't spot (might leave a slight film, but nothing compared to water spots) and dries very quickly.
The factory cover is available through Grubbs or your local dealer. I think it runs about $280, is fitted, light grey and has a nice silkscreened Infiniti logo on the front and clear panel over the rear licence plate. I'll try to put up a couple pics later.