An old line about German philosophy (not quite applicable after the development of airplanes) held that the British conquered the seas, while the Germans conquered the air. Another line was that German philosophers dive deeper and come up muddier. German philosophy, particularly of the idealist persuasion, tends to make up for what it lacks in clarity with quasi-mysticism. This creates a readability problem, to put it nicely.
The readability problem is there with Kant, but everyone sort of agrees that he's being difficult out of necessity. Hegel, who is also hard to read, once said only one man ever understood what he was saying, and even he didn't really understand. But, even that sounds like an inflated number.
Heidegger is perhaps the worst of them, if only because he insists on his own idiosyncratic meanings for words that the rest of us use differently, and then never really makes it clear how he understands the words. What I tried doing, during my own years wandering lost in his deserts, was to simplify every section of Being and Time as far as I possibly could. What I ended up with was a lot of notes that read like they were written by a child and insights into being that I'd already read in Henri Bergson, who was at least able to write without inventing his own idiom. When you point the similarities out to Heidegger fans, they dismiss them out of hand by
pointing out that he famously dismissed Bergson in a footnote in Being and Time. Sure he did- anxiety of influence, I'd imagine.
But who knows? I can't claim to really catch the jive that Heidegger is laying down. Besides, the originality question isn't the big issue with Heidegger; after all, it's a bit overshadowed by the Nazi question. As his scholars like to put it, Heidegger "flirted with Nazism"; but in fact, the flirtation was fully consummated. Heidegger was of the protected mandarin class under the Third Reich and gladly spouted Nazi rhetoric, which just happened, conveniently, to dovetail nicely with many of his own writings aboutpremodern peasants, the homelessness of modernity, and the lost meaning of being in the post-Enlightenment world.
After the war, Heidegger tried to fob himself off as having been quietly at odds with the Nazi regime while rector at Freiburg. After his death, documents started surfacing showing that he got all the mileage he could out of the Nazi regime, including acting particularly beastly towards some of his Jewish colleagues. Heidegger was a Nazi- of course he was- and indeed there are reasons to believe that his own philosophical ideas led him to the political ideology of Nazism. Now, that's not to say that the ideas led him there by necessity- your results may vary. It's hard to think of anyone for whom reading Heidegger led them to National Socialism; however, quite a few were lead to writing almost unreadable dissertations, which is a sort of Nazism in my opinion.
The problem with Heidegger's vagueness is that it takes so long to understand him that once you do understand him, you're no longer sure you've understood him. Thus everyone who critiques him is accused by his celebrators of having simply misunderstood him, and those who love him are accused by his critics of having misunderstood him. There seem to be no middle ground: either he was the greatest philosopher of the twentieth century, or he was a charlatan who really wasn't saying anything much at all. In the Chronicle of Higher Ed, Carlin Romano argues that Heidegger was, "a prolific, provincial Nazi hack," and catches all manner of hell for it in the comments section.
Some have even suggested that we throw out Heidegger altogether because his philosophy smells a bit too much like blood & soil. It all depends on how we understand ideas: some people see ideas like a battery that gets plugged into people and makes them go. I think this is where most calls for censorship come from- a notion that ideas have set & specific consequences. But, of course, they don't. There are billions of possible interpretations of any idea worth having, and billions of different ways of acting upon those ideas, as the history of every religion would seem to show.
The question is not whether Heidegger was a Nazi: he was. It's not if his Nazism was somehow connected to his philosophy: it probably was. The question is whether there's anything to his ideas worth preserving, or if it's all a bunch of vague gobbledygook. I can't even pretend to answer that question; but I would imagine that no matter what the answer, people will still be able to play around with those ideas and make something useful out of them. Again, your results may vary.