Better than you think it is, but not as good as you hope it'd be
Every review will point out that Child of Eden is a graphically updated version of Rez, and it's true. The controls are minimally updated, the overall framework of the gameplay similar... even the overall structure of the game rings a lot of familiar bells. However, I'm guessing that gamers that know Rez well and want more of it already have this game and aren't waiting for reviews. Let me see what I can say to everybody else.
As my own background, I did play Rez - not on the original Dreamcast, but in its PS2 and then XBLA incarnations. And as a musician and deep fan of musical gaming, I appreciate what the game tries to do, enjoyed thumping my way through it a few times, and then set it aside as a fond memory but not much more. I was excited to see that Child of Eden looked like Rez, and excited to try it out.
Control-wise, it's a simple rail shooter where you lock on up to eight targets and then release the button to fire. The right trigger activates an alternate "tracer" fire that's less effective against normal enemies but necessary to take down incoming projectiles, and later on some enemies are specifically only vulnerable to one or the other attack. A smart bomb "euphoria" rounds out the ability set. Surviving a stage requires knocking out anything dangerous before it hurts you, and high scores/achievements often call for killing ('purifying') everything in a level and also firing full eight-target shots in time with the music. There's considerable replay and frustration available for those who want to shoot for the perfectionist end of the spectrum, and the achievement points seem weighted towards the more difficult feats.
Really, if that replayability does not appeal to you, you're going to find this a very unsatisfying game. There are truly only five real levels... a sixth "challenge" level is also available, but will require most players to work at for some time to make headway in. Reasonably, you can expect to play this for at most a few hours until you're beyond seeing anything new. Personally, I enjoyed spending a bit of time working at the achievements and trying to make some headway in the challenge level, but full "gold star" completion of levels looks like a very challenging proposition that relies mostly on carefully executing exact choreography (rather than reacting to situations)... that's a little bit beyond what I'm willing to put into my gaming most of the time.
It's worth pointing out that the Kinect controls for this game are actually rather good, and worth experiencing at least a few times. The hand control is responsive; you switch between main and alternate fire simply by switching hands, which works out mostly well. The reticle is larger and the criteria for ranking at the end of the levels seems to be more forgiving, so it makes up a little for the inaccuracy, but in honest anybody going for the high-end achievements will need to go with a controller. As a final note, the firing gesture - pushing the hand forward towards the screen - can get very wearying on the elbow. It doesn't have to be done super forcefully and the Kinect can pick up the gesture even if it's not a sharp motion. So spare yourself a little pain. The end result actually is reminiscent to the Ubi stage demo a year ago - the player looks a bit like an orchestra conductor gesticulating in front of the screen, and there's something very fun and rewarding in that experience.
Many people will argue that the amount of entertainment you get for a full price Child of Eden purchase is a bad deal, and there's no objective way to deny that. There's just too little content, even though it's good content. A full purchase can be justified if you think of it as a statement of support for Q Entertainment and their distinctive artsy independent creativity. Personally I wish they'd flex that creativity a little more forcefully than they have in recent years.
Overall, it will be easy to dismiss this game, but it's an effective and fun return to Rez's mechanics that's ultimately undermined by its own brevity. I'd suggest renting this game to see what you think, but if you like what the developer is doing, consider making a full purchase simply in the name of putting your dollars where your mouth is and supporting one of the prototypical artsy indie developers.