The graphics and music were really fitting, the gameplay was unique and the levels weren't very frustrating but still managed to have a noticeable difficulty gap. I would go more in detail about this, but literally EVERYONE has covered these aspects already.
The final boss, long story short, is genius. On a gameplay standpoint, not only is the AI unique, the way to defeat him is too. It takes an aspect that hasn't been used that much except for being precise with your "wall jumps", and uses it to the extreme. Nula's got this interpretation covered, so I won't waste time on that.
One last thing. When the Final Boss dies, DECEPTION becomes DEPICTONE (The title of the game), and keeps shuffling until it gets to POETIC END. Just goes to show this game's genius.