Warts and all, 'The New Scooby Doo Movies' (while with hit and miss guest stars, villains and episodes), though admittedly re-watching the show as an adult this reviewer does like it quite a bit less than when a kid, is one of the stronger Scooby Doo incarnations.
So far, the best of the episodes, watching them in order of release/airing, have been "The Dynamic Scooby Doo Affair" and particularly "Wednesday is Missing", "The Frickert Fracas" is also very entertaining. "A Good Medium is Rare" is perhaps third weakest behind "The Ghastly Ghost Town" and "Guess Who's Knott Coming to Dinner". Just for the record, this reviewer doesn't dislike any of these.
"A Good Medium is Rare" has some great moments, such as agreed the very creepy and entertaining if underused demon creature and some of Scooby and Shaggy's antics early on. Shaggy and Scooby are a joy, as is their charming, amusing and affecting friendship, while Phyllis Diller's guest appearance is a lot of fun with some of the best and funniest lines of the episode.
In fact, the dialogue is endearingly silly and is very funny often and even more so on occasions. The voice acting is very good, from Don Messick, Casey Kasem, Frank Welker and Diller especially.
On the other hand, the story isn't so strong here. The episode does feel too long and too stretched, with a story that has its moments but lacks tightness in the pacing and a quite thin mystery that lacks originality and even fewer surprises.
The rest of the gang are a touch bland this time around, and the villains are okay but not that memorable. 'The New Scooby Doo Movies' was never known for great animation, often it was the worst thing about it, and while there is worse in the series it is definitely less than great here, too often looking flat and crude.
On the whole, decent episode but one of the lesser episodes of the show so far and a bit dull. 7/10 Bethany Cox