The first two seasons I'd go at least for an 8/10, when it comes to rating this. The third season though is quite the downfall (or as the term goes, jumps the shark). Whle it still isn't a bad show (and for German standards it's almost phenomenal), it does lack the drive and hillarity of the way this started. Taking cliches and turning them upside down more often than not really works wonders for this. While there are still moments where this may be called misguided and uneven (depiction of certain people or nationalities), this is not meant to be seen as too serious.
Something that is or gets way over the top by season 3, where it loses touch with "reality" many times. One of the German TV shows that was ever produced and shining a much needed light on a much maligned and often underrepresented group of people, this almost does some social work ... and was quite the phenomenon when it first aired ... and still finds new fans I reckon! And why not ... just don't expect much of the last season