Rothenburg ob der Tauber is definitely worth staying a night. In fact, they have some interesting night time tours of the city offered by the tourist office. It's one of the most visited medieval cities in Germany.
Getting there from Mainz to Rothenburg is a bit difficult. Best you get the exact directions from the website of the German railway, which is also in English. Refer www.bahn.de.
It takes 3 - 4 hours, and requires changing trains in Frankfurt and Würzburg and possibly Steinach. Then to go on to Munich, it takes about 3.5 hours. Go back to Steinach, then to Würzburg and then on to Munich. Have a look at it on Google Earth.
Have fun. It's worthwhile!
Reisender
QUOTE(germany2011 @ Aug 23 2010, 09:53 PM)

How do I get to Rothenberg ob der Tauber by train from Mainz and how many hours does the journey take? And, would be advisable to stay the night or can get to Munich easily from there? Thanks.