Having never played a Ratchet game before, I wasn't too impressed with the demo, but I decided to give it another try and make it my first Gamefly PS3 rental (there was nothing else, haha). I'm actually glad I did, because I've been enjoying it. Yeah, it seems pretty familiar to other platformers I've played, but the characters are charming, and the graphics are good for the most part. The Sixaxis controls make sense where they're used, and are responsive. Not the best game ever, and I wouldn't ever buy it, but it's been a really fun rental so far.
If a Rock Band purchase doesn't work out for me (I'm already starting to develop a "fuck that game" attitude towards it), then I might use that money to show the PS3 a little love and get Assassin's Creed for it, and Mass Effect for the 360.