There's not a single comment (so far) for 'Tatort'? That's really hard to believe, since it's a very long running and popular series here in Germany. I have seen some episodes, but not enough to consider myself an expert on the series. If you are an expert, please go ahead and add another comment.
Normally I always include a rating for the titles I comment on, but I think it would be unfair to do so in this case. I don't think I have seen any of the episodes from the 70s except for 'Reifezeugnis', and I haven't seen many of those of later years. I don't consider myself an avid fan of the series.
So what can I tell you about 'Tatort' ('crime scene')? The basic idea is interesting. There are about 35 episodes per year (at least nowadays). But you don't get to see the same team of detectives in every episode. There are different teams from different parts of Germany; that's what makes it interesting. (Some episodes even feature teams from other countries, namely Austria and Switzerland.)
But I think that, at least nowadays, too many episodes suffer from bad scripts. Often the cases are too confusing because they cram too much into the 90 minutes running time. If you are interested in examples, go to the user comments page for the episode 'Das Böse'. I have written a comment on it. There's also a somewhat similar comment for 'Der Kormorankrieg', titled 'Execrable' and written by a user called 'moabitnik'. (Please keep in mind that user comments are subject to change and deletion. Though it's unlikely, a comment might be different or gone tomorrow. I only refer to other comments because I think it would be redundant to repeat information that can be read elsewhere.) And I think that in many scripts there's too much 'political correctness', for example there are too many iffy 'power women' among the detectives.
Some more information that might be of interest:
Some episodes were only shown in Austria. They are listed as a spin-off on the 'movie connections' page for 'Tatort'.
'Reifezeugnis' is perhaps the most famous episode. It was directed by Wolfgang Petersen of 'Das Boot' fame and stars a young Nastassja Kinski.
I guess Schimanski, played by Götz George, is the most popular 'Tatort'-detective. He got a spin-off series and two theatrical movies were made with him.
The German experimental rock band Tangerine Dream wrote the title track for 'Das Mädchen auf der Treppe' and scored a hit with it in Germany.
Some episodes are no longer shown as reruns because their content is deemed too controversial. Some episodes have been edited after their original airing, for example because of unwanted product placement.
Every German who has a TV set is required by law to pay fees for the station that produces 'Tatort'. Believe it or not, you have to pay for it even if you don't watch it. So the makers don't have to try hard to get funding for the next episodes. I guess the quality of the series might improve if they had to try harder.
Based on the episodes I have seen in the last few years, I would give 'Tatort' a rating of four points out of ten. But, as I have mentioned above, I might be wrong, since I haven't watched many of the episodes, so I won't rate it.
Edit / addition (27th August 2008): In the last months, I have seen some reruns of old episodes from the 70s and 80s, and some new episodes. There were a lot of good episodes among both of these, so I would now give the series a rating of six points out of ten, because I now guess that the ratio of bad episodes is lower than I had thought before.