Those who have purchased and installed iMovie ’08 and have an older G4 based Mac you cannot use the newer iMovie ’08. So Apple has made available its iMovie HD 6 product for download so you can still get your iMovie on.
I think this is smart of Apple, not to build a product that many of its customers can’t use, but to make this available to those who either don’t like the new iMovie, prefer the old iMovie, or can’t use the new iMovie.
Another thing to mention is that the new iMovie ’08 does not have any plugin support so all the plugins that have been made for iMovie HD 6 won’t be any use to you with iMovie ’08 (just another reason to download iMovie 6 if you purchased iLife ’08).
