Well, I've only played 3 or 4 songs on Helix, but my shoulders are tired (should have been keeping up with Wii Fit, and this wouldn't be a problem), so here are my very early impressions. If you haven't seen any gameplay, watch this to get an idea of what I'm talking about:
http://www.youtube.com/watch?v=_tlirweG7KY
I played one song on easy, but that was a little too boring, so I bumped it up to medium for the others. The problem with the game is exactly what I feared. The timing bar doesn't have enough information in it (when to move, but not what to do), so it's very easy to get confused about what you should be doing. You have to watch the moves as the robot performs them, sometimes 1 or 2 seconds before you're supposed to be doing them, all while doing the moves that the robot did a second or two ago. The result ends up being a rhythm game that you play off beat, and it just feels... weird.
Now, for the most part, so far it hasn't been too bad. But the times when you're likely to screw up are when the robot changes it's starting side (left, right, left instead of right, left, right, for example), or when it throws you a completely new move (like a forward circle, or the double-handed "S" move). As a result, you might say that it's like a physical version of Brain Age, really testing your visual processing, hand-eye-coordination, and memory all at once. It's a very different type of rhythm game.
If you decide to get it, I hope you like techno. I haven't listened to all of the tracks yet, but flipping through, they appeared to all be techno. The techno track in Donkey Konga 2 (I think it was 2) was one of my favorites because it was really tough. I'm not sure I can handle 26 techno tracks.
I want to put in some more time with it this evening and tomorrow, to see how well I can adapt to the inherit challenges in the gameplay. I'll try to post more within the next day or two.