Dwight Frye

A lot of it will depend on what your requirements are for a cover. I need one to cover my car during the day in an outdoor parking lot in the SouCal sun. The car is garaged at night. I had a Covercraft Weathershield cover for my Infiniti G35 that was used almost daily for 4 years when parked outside at my place of employment. It did a really good job of keeping the dust off the car as there was a lot of construction going on nearby for about a year. It's also breathable and doesn't trap moisture underneath and was effective at keeping the sun from beating down on the paint and upholstery. The trick is keeping a good coat of wax on your car and using something like a California Car Duster to remove dust before covering it. The Covercraft covers are custom fit which in my opinion is important in breezy conditions. The generic size and cheap material covers will buffet and rub on the paint on any high edges. The Covercraft Weathershield also will roll up into a very compact package, important in a car like a Z with very limited storage space.
That model is not weatherproof, in other words not for use to keep rain off of the car if it's not to be driven for a while for example.
I just bought another Covercraft Weathershield for my 370Z and it fits perfectly. I expect similar good service from it. The downside ? They are not cheap. I paid about $275 including the little storage bag.
Check Covercraft's website, it has pretty good comparison charts for it's different models showing what their strengths and weaknesses are depending on what you need out of the cover.