Heart of Darkness is about a 7.5 game with 11 atheistics. The pixel art and astoundingly detailed and fluid hand-drawn animation would make it worthwhile nonetheless, that its also a pretty good game is a bonus. But of its particular sub-genre its not at the cream of the crop (its behind Flashback, Blackthorne, Oddworld), the controls will fail you and its absurd artistry is matched by a similarly high and evil difficulty. The CGI cutscenes kinda suck too.
But still, its got stupid levels of gorgeous. I don't think there's a 2D game of the PS1 era that's nearly as technically accomplished. Hell, its a strong contender for the best EVER cel-animated game. Its that good. Really.