I would give it a 6. In the later levels it gets a little tricky when having to switch between light/dark while jumping multiple times but it's not frustrating.
It's actually very rewarding when you get the flow of the game. The boss fights are well thought out and interesting with only a simple pattern recognition to defeat them. The last boss combines everything you have hopefully mastered through the course of the game and is challenging without being annoying. Basically if you fail anywhere it's due to a mistake on your part and not some cheap game tactic.
Also, checkpoints are scattered pretty liberally so you are not punished too harshly for death. Overall this game is very much worth the playthrough I just wish there was so much more to it. I finished the game, got all achievements and wanted more but could only start over or try perfecting the timed runs for each level.