This series holds up surprisingly well!
All right, I'm a total nostalgia hound for almost anything from the 1970's... I was born in '72 and have very fond memories of all things 1975-1979, including THE HARDY BOYS. I've been re-watching or buying (or both) a bunch of old shows from the '70s that I grew up on and it's a little disheartening to see that old shows like SPACE: 1999, WONDER WOMAN, BUCK ROGERS IN THE 25TH CENTURY, BATTLESTAR GALACTICA, WELCOME BACK KOTTER, et al, are actually kinda crappy. The nostalgia factor is sweet, but sometimes trudging through some of these episodes can be a real chore and eye-roller.
As much as there is a definite '70s-cheese factor to THE HARDY BOYS/NANCY DREW MYSTERIES, I have to admit that while they never broke any new ground dramatically or technically, most of the episodes hold up surprisingly well and are solidly watchable. You get sucked right into the mystery, you chuckle at the lame one-liners and '70s fashions, and you stick around to see how it all ends.
The only other '70s show I'm currently watching on DVD that seems to hold up just as well is the Bixby/Ferrigno INCREDIBLE HULK.
Good, harmless fun. Hopefully my daughters will be fascinated with Parker Stevenson and Shaun Cassidy some day and watch them with their parents!