Rick Steves mentions that the Gutenburg museum is not worth going to. However, I'd like to mention that it is well worth the visit if you are in the area. There are three levels of artifacts, a 20 minute film background opportunity, and a Gutenburg printing demonstration - all for 10 euros. The museum includes a museum of the newspaper influence in Europe and a book binding museum. China, Japan, Egypt are all represented as being part of paper, print and the written word of communication. I highly recommend this museum. In the same area there is the Roman-Germanic museum (free) and a ship-building museum(free), and numerous roman catholic and evangelical churches to see.
I think Mainz is a great town to visit. The cathedral is one of the most important ones in Germany as the Archbishop of Mainz had considerable power in the electing of the Holy Roman Emperor. It is 1000 years old and when you walk inside, the massiviness hits you. This is one of my favorite churches in Germany. Make sure you find the baptismal font, as it is beautiful.
My husband enjoyed the Gutenburg museum too. There are also quite a few Roman ruins in Mainz and they are constantly working on more. The alt stadt is lovely and is perfect for strolling around.
http://www.mainz.de/WGAPublisher/online/html/default/akah-67wd32.en.html
Also, Augustinerstrasse with the Baroque church is a great place to visit. Mainz has some of the friendliest people we met on our last trip. The market is also wonderful, on days when it operates.
http://www.germany-tourism.de/ENG/destination_germany/master_tlstadt-id1138-fstadt_allgemein.htm