I was in Hannover in 2005 on a business trip to attend a CeBIT Conference.  Outside of the CeBIT , there is zero reason to visit Hannover, and recommend you not waste a single minute of your time there unless you absolutely have to be there.  It's a very, very dull, City.  To most Germans, Hannover is just a train stop along the way to/from Hamburg.  Nobody willingly visits there.  Go to Berlin, or even Munich, but certainly don't willingly go to Hannover unless you need to catch up on your sleep.