As a German I can assure you that I can not understand anything (yet). Some words sound very familiar, but the vast majority does not, which makes understanding pretty much impossible. I mean there are words in english as well which sound very close to their German counter parts (like house (Haus) and mouse (Maus)), but that doesn't mean that we can understand each other. I'm pretty sure that goes for almost all languages. :)
But I can see why some people might think that way, since they both sound somewhat alike (like most Germanic languages).