When are Hitler/Nazi comparisons appropriate?

In recent years, anyone who made a reference to Hitler or the Nazi party to criticize a political opponent has been roundly denounced and rightfully so. It is ridiculous to compare a budgetary decision or other policy matter to a systemic plan to exterminate a race of people. Unfortunately, as with a lot of things the knee jerk reaction is it is never right to make the historical comparison to the evil committed by Nazi Germany in the 1930's and 40's. In Suzanne Field's column this morning she discusses one reason why it is important to sometimes make that comparison. If people had read and understood Mein Kampf then they knew what Hitler intended. Similarly, there should be no doubt today as to the intentions of Hamas towards Israel.