If you compare anything, anything at all, to Hitler or Nazis, then you are a goddamn idiot.
It's not that there aren't valid comparisons to the Third Reich. There absolutely are. Mussolini, Hirohito, and Franco are all great places to start. And there are certainly analogues to the Holocaust in world history: the Spanish Inquisition, of course, but also the reign of the Khmer Rouge, the Rwandan genocide of the early 1990's, and even what's going on right now in Darfur all bear marked similarities to the Holocaust. But bringing up these other genocides would at least be a sign that you'd done your homework. Everyone knows about the Holocaust; the fact that you can make a comparison between some event or practice and the systematic extermination of approximately 14 million human lives is not impressive, and most likely it is a grossly inappropriate comparison.
Comparing something to Hitler or Nazis is such a problem in rhetoric that there's actually a theorem called Godwin's Law which states that "As a Usenet discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches one." Eventually, someone is going to make the comparison, and the discussion is essentially over at that point.
Once the subject of a debate has been compared to Nazism, there's no hope for the continuation of cool, rational debate. Nazism is evil, and no one can save face while appearing to defend it, or anything associated with it. The subject so tagged has now gone beyond the field of debate and into metaphysics, perhaps even theology, judging by how fiercely people defend against their pet ideologies being compared with National Socialism. Essentially, the person who proves Godwin right has effectively declared that "viewpoint X is evil, all who espouse it are evil, and any attempt to defend viewpoint X is the equivalent of a hate crime." Once somebody's tossed out an ultimatum like that, it's no wonder that the conversation has nowhere to go but down.
But the most important reason to avoid comparing anything to Hitler or Nazis is that it makes you sound like an idiot. It really does. Even if you're making a totally legitimate comparison between the practices and propaganda of the Third Reich with some equally villainous organization (for example, the Taliban), the act of comparing anything with das Vaterland is so tainted by its association with spluttering, indignant maniacs that even legitimate comparisons suffer for it. Even if the comparison is valid, well-considered, and backed up by data and historical evidence, anyone listening is likely to be on their guard against such comparisons, will assume that such comparison is being made falsely, and will react just as one might predict. Mentioning Nazis chenges the whole tenor of a debate from an attempt to arrive at truth to a desperate struggle to crush the forces of evil under a merciless barrage of logic and namecalling.
In debate as well as in life, just stay away from Nazis. Nothing good can come of inviting them to come in.