Ok, so not the worst movie. Probably the worst Scooby Doo movie out there, but at least it's still watchable. The guest stars and side characters have more likability than the main cast, sans Velma who absolutely dominates, to the point where the movie is almost about her. Bill Nye was an absolute treat, as wasn't Elvira, both of which are extremely likable and quirky in their own ways here. Shaggy and Scooby get dummed down a bit, but at least they're still funny. It's Daphne and Fred who really get nerfed, with the former being absolutely unacceptable (Fred was already a bit to late to be saved.)
The villain is good, with a rather good twist and is still scary enough to keep you on the edge of your seat. Although there are some references in there that one would swear are just to prop up the movie.
Overall, watchable, with this really being Velma's time to shine, but don't count it as a must watch.