I've beaten it. Overall I'd say it's on par with the first 2, with some things improved and some things not necessarily changed for the better. That means it's still at least a 9 out of 10, but I have some criticisms relative to the quality of the series so far.
I should have toughed through it on Hard, since after Bryyo the normal difficulty was way too easy (compared to Prime 1 and 2). The final bosses barely got a hit on me.
The majority of the voice acting was very enjoyable, but the implementation of cinematic elements was uneven throughout the course of the game. There are long stretches of several hours where you begin to feel like you're back in the old Primes, then suddenly you're knee deep in "interactive storytelling" again, interacting with static NPCs.
There is a ton of exposition and setup up front, with almost no payoff at the end. This is the same pattern as Prime 2's attempt at story, with all that detailed Luminoth backstory leading up to a complete non-ending.
I'm not sure I like how the seperation of areas by landing pad has changed the game. It makes backtracking a little easier, sure, but the planets are no longer one big connected world. Level designs have de-emphasized platforming, since rooms no longer have to be that interesting or fun to travel through over and over. This decompression has it's unique qualities, but I've found these worlds to be the least memorable in the Prime series.
(Minor spoilers ahead.)
The puzzles this time focus on switch flipping. Granted, the idea of constructing a huge bomb piece by piece is cool, but you are still basically flipping a series of switches in a particular order. It's all very elaborate, using the wiimote to twist and turn, timed target shooting, alternate visors, etc. It's not my idea of improved gameplay, though.
I'm a fanatic for morph ball puzzles. Unfortunately, this game does not raise the bar set by Prime 2's puzzles. There is nothing as crazy as the morph ball boss fight or the timed platforms in the Sanctuary glass case. There are certainly some good morph ball moments, but they are basically remakes of previous puzzles, like crossing the gap to the Sanctuary entrance.
It's a great game, I'm just dissapointed the core platforming gameplay had to be pared down for the sake of the wiimote.