Well after playing the game for a few hours I can indeed say it rocks. Even though I've only played through the first worlds levels and some of the second worlds, the gameplay is top notch. The levels are nicely designed with Treasure to discover, some of it cleverly hidden. Plus the soundtrack is F*CKING AMAZING.
Which brings up my next point, the missions for each level really make the the game shine. Even in the first few levels, your required to really learn about the level before being able to complete some of the missions, and will for sure play some of them several times before you accomplish them.
Any reviewer that calls this game too short and easy, is a f*cking failure as a gamer. Every level is filled with Treasures to discover and missions to complete, with some of these missions quite challenging. I'm already close to 3 hours in and I still haven't completed all the missions in the first worlds levels yet. The way things are going I easily see this game taking somewhere over 20 hours to complete everything 100%.
Now the normal levels themselves have been easy if you just rush through them without doing any of the missions. It's obvious that this is what the reviewers have been doing because yes, I'm sure it's possible to get to the final boss in about 5-6 hours if you play like this. But the depth from the gameplay comes from finding the hidden Treasures and completing the Missions. Anyone that plays through the game without doing any of them isn't playing the game the way it's meant to be played and as a result is a failure.
From the looks of things this game looks like it'll end up the Wii's Jungle Beat. A top notch 2d platformer with tons of value and depth to it, that gets low reviews because the reviewers were complete idiots that didn't bother to play the game the way it's meant to be played.
For everyone else though, that actually plays through the game the right way by trying to find all the Treasure and completing all the Missions 100%, you'll have a blast.