I hope I'm not behind the times on this, but EB/GS has a playable demo of Prime 3 up. I played it for over an hour (the store was mostly empty besides the girl working there) last night, and here are my impressions:
Before I begin I should mention that I thought Metroid Prime was one of the finest games of the last few years and I thought Prime 2 was even better. So I had awful high expectations going into Prime 3. So here goes...
The first thing that hits you is obviously the controls. And to be perfectly honest, I didn't like them as much as I was hoping to and felt like they may be a step back for the series. Previously, you could lock onto an enemy then just hammer the A button and shoot right at them, but now you have to lock on AND keep track of your aiming reticle (which was easy to lose track of in my opinion). So aiming is now harder as you have to multi-task, however it is very rewarding when a nice big charge shot hits an enemy point blank. I also thought that aiming with the wiimote is a bit hard because it's easy to go offscreen (or off sensor) and then you'll have to get yourself reset. By the end of the demo I found the controls to be a bit better, perhaps they just have a steeper learning curve than I had expected, but I don't think I could quite call them an advancement. More just different, but equal at best.
The other thing that you notice is that this is the first Metroid game to have NPCs and voice acting. You start out on a Federation ship and you can talk to people with uniforms that wouldn't look out of place at a Star Trek convention. Personally, I felt like this was an awful decision. One of the most endearing aspects of the Metroid franchise is its total sense of haunting isolation where you are a lone human on a hostile alien world. By bringing in other characters it feels a lot more generic, made doubly so by the uninspired design and dialogue of these talking characters. Hopefully this is only prevalent in the early parts of the game, but it's a big step in the wrong direction in my opinion.
I found the early game environments to be both bland and way too dark. It's easy to get lost running around. (luckily the map is still excellent.) Screenshots I've seen from later in the game indicate that the environments will become more creative later, but I didn't think it was a good intro.
But all nitpicking aside, did I think it was a fun game? The answer: hell yeah! Metroid is still Metroid and it's a tough thing to ruin. The two boss fights I played were excellent, the pacing was real fast (after the horrid talky intro), and the combat is still a lot of fun. I don't know how I feel about traveling to different world, but that's something that I'll only be able to determine upon playing the full version.
Ultimately, while I felt this was a step in the wrong direction and subpar for a Metroid game it still felt like a good game in its' own right. Think of it as being the Mario Sunshine of Metroid games. Now Mario Sunshine wasn't a bad game by any means; it just wasn't as good as Mario 64 from a few years before. I would have given Metroid Prime 1 & 2 a 9.5/10 and based on the demo Prime 3 seemed to be right around the 8.5/10 range. I'm still going to continue with my pre-order and still think it will be a lot of fun, but those were my initial impressions anyway. But maybe it's good that my expectations have been lowered a bit, so the full version will have a higher chance of exceeding them.