GamesRadar has done a preview of Metroid Prime 3 and has a video showing the first hour of gameplay. There aren't a whole lot of spoilers if you've seen the previous MP3 video previews, but all the same you should know that spoilage is possible. Also the person playing the game sucks horribly and there are some spots where you may get a headache or worse if you suffer from seizures.
Like I've said I haven't played the previous Prime games, but I am definitely liking what I'm seeing. There is a lot of dialog from the various crew members and the interaction with the environment is great. It's unfortunate that gamers who can't let go of the past don't like these changes, but I'm glad that they are here and can only hope for more - Like Samus finally getting a voice actor and dialog. People complain that these changes make Metroid just another generic FPS, but Metroid can never be generic unless they drastically alter the gameplay - and they haven't done that from what I can tell.
In any case here is the link if you want to see the first hour of Metroid Prime 3