He can boast of having taken all four of his club into Europe, which is quite the feat, considering he took over at Köln and Aachen while they were in the second division and Hannover/Wolfsburg while they weren't in amazing shape. So he had (some) success wherever he went. On the other hand he's very outspoken and can clash with people, he didn't have a bit of an ugly end at Köln and went through quite a few coaches at Wolfsburg.
The biggest club he's been in charge of was Köln or Wolfsburg (depending whether you look at money or public interest). On the German spectrum I wouldn't have been surprised to see Gladbach show interest him, but I don't think Bayern or Dortmund ever would have considered him. So it's a bit surreal to see him get a big post at Liverpool, as it has to be an entirely different world to what he's used to in so many ways.