I just saw this and I thought it was rubbish, but my kids loved it so I guess that gives it the seal of approval.
My son (aged 6) liked the Scooby and Shaggy farting and burping competition (I knew he would have liked 'Thunderpants'!) I'm not sure they followed the disembodied spirits storyline though.
Velma did come over as the coolest character, especially when she swapped her sweater for the low cut dress, but I didn't think the guy with the 'Led Zepplin' T-shirt was her type.
I have to say the Freddie Prinze Jnr. had the Shaggy accent just perfect though. I didn't see SMG as Daphne, and I don't watch 'Buffy' either. Fred didn't look right at all. Shaggy in love was too much to bear though.
They do split into groups, but Velma objects to the Daphne and Fred split, so she goes with Fred. Daphne has got sick of always having to be rescued, so she has learnt some martial arts defence moves since they last went on an investigation.
And Scrappy Doo has a much larger part that I was expecting.
The interesting thing was that when we got back from the cinema the 'Cartoon Network' channel (which is showing back-to-back Scooby Doo all day, all week) had a 'Scooby and Scappy Doo' episode on which must have served as a source for the film story. We spotted more than a few ideas that were in the film -- mixed up identities being the main one.