Best advice from this compadre is to just use some mild soap and a soft wash mit for the top. Less expensive and works better...just my .02 and I own a 97 droptop.

Word of advice to those of you who have recently purchased your droptops, get the 303 High Tech Fabric Guard ( $16.95 from the Corvette Museum www.corvettemuseum.com ). Get the roof cleaned well...and spray the fabric guard on as directed to protect your roof from weathering and staining. It's great stuff! I just wish I knew about this stuff sooner.
 

I used to use soap and water and it worked well. I have the Haartz cleaner now and it works well but I don't think it does any better than the soap and water. My advice is to invest in a good soft brush so you can clean the top well and get an Absorber or something else that doesn't leave lint when you dry. I don't think the soap/cleaner you use is really the most important thing. Like Gonzo said, use the 303 Fabric Guard. That is a must!