I have one on mine...
They aren't original in terms of original from the factory, they were mostly a dealer installed option. Mine has a British canvas roof just like they installed on some MG GT6s. It is bigger than a sunroof and way cooler. It folds back like an accordian (as per previous post).
My 2 cents? I nearly didn't buy the car when I seen it. Then I figured, well, I can always replace it with a sunroof. First couple of sunny days and I was sold. It works great and is way better than a sunroof - it opens the car up completely! As for the looks? Well I personally think it suits the vintage of the car, it closes tight to the roof so it doesn't screw up the body lines. Mine is only the second Z I have ever seen with it (the other Z had an indentical roof) so like or not, it is definitely a conversation piece!
I think you'll come to like it so I wouldn't let it put you off. Mine isn't in bad shape but I intend on having it recovered when I completely redo the bodywork on my car.