These are worth it if you want to visit Palazzo Ducale, Palazzo Te and Teatro Bibiena.
There are 2 options, the Mantova Musei Card 5 (15 euros) and the Mantova Musei Card 8 (17 euros). Both are valid for a year from the date of purchase.
The Card 5 gives free entry to Palazzo Ducale (normal admission 6.50), Palazzo Te (admission 8.00), Teatro Bibiena (2.00) and to two smaller museums.
The Mantova Musei Card 8 covers an additional 3 museums, including the clock tower with view of the city (normal ticket price 3.00).
Full details of Mantova cards in Italian can be found at:
You can buy them at all participating museums/attractions.