I wasn't sure whether to buy this movie. It's a made-for-TV "prequel" to the other two movies, with different (younger) actors in the title roles. It could easily have been awful.
Actually, it turns out to be quite good. It has a silly plot as usual - that's part of the fun of Scooby Doo - but the action moves along nice and swiftly, and the young actors throw themselves into the silliness with enthusiasm. Hayley Kiyoko, who plays Velma, is particularly worth watching. The special effects are pretty impressive for a relatively low-budget movie.
The extras are pretty awful (a music video, a not-very-good personality quiz and a couple of gag reels).