Let me start with a disclaimer - if you are not very comfortable with your German, you'd be best served to avoid this book altogether - it might still make you laugh but it will also confuse as to what's the right way to speak the language and what not.
Sick's weekly contributions from SpiegelOnline are summarised in book form here, so if you have been reading those, the book is likely to be superfluous. If not, however (or if you are too lazy to try and find all of them), the book is a hillarious debate about the degeneration of the German language, especially when used in advertising, by politicians or the press. Here Sick chooses the lighter route of the sarcastic observer, rather than the unreadable alternative of the righteous crusader of the correct German language.
His command is impressive and he is an advocate of German in a similar fashion to the job done by people like Stephen Fry for English. While one will not find a treasure trove of new vocabulary in the book like one would with Fry, Sick does ensure that one at least knows how to navigate the morass of German grammar a bit more reliably after reading the book. To top it off, the book is a laugh a minute, too.