I've completed the game now (credits rolled) and am now doing the lost levels and whatever else is needed.
Good game, but for me it really doesn't have a clue how to increase the difficulty spike in a way that makes it enjoyable.
So far, all it does is throw a well placed enemy in your path that makes things difficult. Just results in the level having to be replayed 50 times and if you do manage to do it, its often down to luck rather than any sort of skill. I've found this on a few occasions so far, specifically the X, Triangle, O and Square levels. It's just a cheap way to give perceived difficulty rather than having you work for it in a rewarding way.
That aside, good so far but it's nowhere near game of the year for me.