It's awesome as long as your kid can handle the monsters.
I waited a long time (age 6) to allow my daughter to watch this series. She is mildly sensitive to scary stuff and I thought this would be too much until then. Turned out I was right on, though there is still one episode I won't let her see (a ghost/monster that comes in people's bedrooms as they sleep, and he makes a pretty awful sound too). Most of the monsters are pretty tame though.
One other thing, there's a lot of talk about death, graveyards, tombs, coffins, mummies, that sort of thing. I have already had to talk a lot more about that stuff than I wished to at her age because of Scooby Doo.
But really, compared to the rubbish they put out these days, it's a wonderful 70s oasis! Also, check out the Scooby Doo Show series and the series with Scrappy (if he doesn't drive you crazy--I'm ok with him). Oh, and for us at least, the original Jonny Quest made a really fun follow-up "mystery" series.