I first saw the show as a kid - just as most others posting on this site, I guess - and I really liked it. The show combined both the slighty scary and the crime stories.
Of course after a while you realize that the stories will always run more or less the same way. There's something supernatural (a ghost, a monster or something like that) and in the end it turns out that this thing has been man-made (usually some bad guy in a costume). That doesn't mean the stories weren't good. And in addition you could guess who was behind it and how it was done by following the clues. For someone like me who wanted to be a p.i. some day (didn't become one, though), that was fun.
Considering the time at which the show was made, the animation was quite good. You cannot apply modern expectations to a series that's as old as you or even older. But the characters were well defined and the stories were firm, at least they never cheated and 'invented' some new evidence in the end as some other shows did.
Even today I enjoy watching the show when I happen to find it running somewhere. To me, it's one of the real classic cartoons and it always will be.