If you play Super Munchkin with one Class, you get all the advantages of being that Class (the ability to equip Class-only Items, monsters with penalties against that Class suffer those penalties) and none of the disadvantages (you can equip Items forbidden to that Class, and monsters do not get bonuses because of your Class).

If the Class has an ability that has a cost, however, you must still pay it – you aren’t that Super!

If you play Super Munchkin while you have two Classes, you have all the normal advantages and disadvantages of both Classes. (All of the above is also true for Half-Breed, just for Races.)