Full disclosure: this school gets points for proximity to my apartment, and I can't compare it to…read moreother language schools in Munich; however, I've taken German in San Francisco (at two different schools), Düsseldorf, London, and Münster (shouldn't I be fluent by now?!) as well as company-sponsored lessons here at my office, and the instruction I received from Andreas at Klartext is really unparalleled. In my experience it's easily on par with instruction at the Goethe-Institut.
That being said, although MY instructor was diligent, thorough, dedicated and uncompromising, over the course of my several months at Klartext I encountered a few substitute teachers who unfortunately were not of exceptional caliber; quite the opposite, really. I even left one class during the break because it felt like such a complete waste of time. My impression was these substitute teachers were not employed by Klartext full-time, and were just available local German teachers stepping in.
Other classmates of mine reported mixed experiences (some superlative, some disastrous) with their previous full-time instructors at Klartext, but all said their feedback at the end of the courses was absorbed and considered by the administration.
I'll also say that you should expect a lot of homework at Klartext - which, in my opinion, is an effective learning method, and therefore an advantage. The level of accountability could be daunting - sometimes after a long day of work you don't want to be scolded and humiliated as you attempt to correct a blank worksheet in front of the class - but it's actually worth it.
I progressed more quickly and significantly under Andreas' instruction at Klartext than anywhere else, and really miss the methods (diagraming on the whiteboard, consistently teaching articles when introducing nouns, doing grammar drills on worksheets, etc.) now that I'm taking classes with a much more "relaxed" instructor.
TL;DR: Don't judge their terrible website: if you're serious about learning German, this school is the place for you; the value for money is excellent. However, if you slack off, your classmates will be annoyed, your teacher will shame you, and you'll likely fail the challenging exams at the end of each session.
I plan to return at some point to further my study... and hope I'll be lucky enough to get another fabulous teacher.