A true classic and without doubt one of the best known German 'Krimis'. I like the episodes from the 70's and early 80's best. Not only because of the sometimes hilarious nostalgia, but the episodes were - in relation to to the later episodes, that is! - fast-paced, and much less boring.
In the 90's, almost every episode concluded in philosophical chatter and most killers were obsessed with "the beauty of death". The series moved away from the whodunnit genre, simply because viewers could no longer guess who was the killer. Even the least obvious characters turned out to have a dark side. The nicest, least suspicious people suddenly turned out to be killing lunatics with no motive but some philosophical crap, and Derrick seemed more of a shrink than an "Oberinspektor". Also, the series moved from the people-next-door environment, mostly in the 70's, to the rich people exclusively in the 90's, which made it sometimes repetitive.
Nevertheless, if you like detective series, whodunnits, and last but not least the German "Krimis", I can recommend this, along with "Tatort" and the "Der Alte" series as a close second. These three were the most popular German cop shows in the 80's and early 90's.
