GRIN brings something different to the table. It's easy to compare it to DREAD as it's a horror game, where the mechanics imply "Succeed until you die or go crazy." It's only one page of rules, just enough for you to grasp but lite enough where you can't help but want to tinker and expand it some. This wasn't a bad thing for me, as I like "building up" with my rules.
If you grab the additional supplements (Torchlight and Asylum) you get a glimpse at how to modify the system for the story you're running.
But focusing on this core document on its own:
-> Really concise, everything you need rules-wise in a single page.
-> Narrative Horror RPG that has more in common with games like Fiasco, where it's intended more for one-shots than campaign play. Also the players (in these rules) don't really have a character sheet aside from a name and a concept.
->Mechanics are a risk-taking "succeed or die." There's a buffer in the way you shuffle the deck to ensure there's game time before tension strikes. You'll find you and the players counting down cards, wondering to odds of each pull as the discard pile becomes smaller than the draw one.
I give it 4/5 because it's GREAT for those who are good at improvising and house ruling. Folks who need more guidance or inspiration will probably be turned off from the game. But with a good group it's really fun, and really intense.