when i first played the game, i put it away after about two hours and didn't look at it again for about two weeks. i just felt like the game wasn't really finished, i was distracted by things like the ridiculous hair design and the way plants look. also, the cutscenes weren't all that motivating for me because of inconsistent character artwork and animation of also inconsistent quality.
but when i picked it up again and decided to go through it at least once, i somehow got hooked. i got used to the hair and the plants and appreciated the beautifully designed environments even more than i did before. and after a couple of hours, once i really (well... kind of...) got the hang of the controls, i noticed how well done they were.
after seeing the (unfortunately not really spectacular) ending, i thought i'd give the time trial mode a try (usually i don't enjoy time trial...) but instead of just trying it for a couple of minutes, the minutes turned into hours. of course, you'd have to be into score hunting to enjoy that because the only bonus you get are star ratings.
by now, i've read a couple of times that the game's difficulty is supposed to be pretty steep at times. i have to say - i don't see it. of course, some of the time trial times are pretty crazy but after playing through on "normal" difficulty and spending maybe 2-3 hours on time trial, i didn't have much trouble playing through the story mode on "hard" without ever shooting an enemy.
if you're someone who only plays games for their story, i'm guessing the rating you'd give this game would be more like 7/10 but if you enjoy high score challenges, i'm pretty sure you'll play the game for a lot longer than just once through the story mode.