I got to play a test of this game once and it was amazing. Partly I was playing with really cool people, but it is also a really, really good game.
For years I've loved stealth video games. Metal Gear: Solid, Tenchu, Splinter Cell, Thief... all of them were good in their own way (at least in their time). They all captivated me for one reason or another, and the stealth was at least enjoyable, if not the primary draw.
With roleplaying games, I've always felt that the mechanics of stealth were lacking. It doesn't FEEL like stealth, it feels like every other mechanic used to resolve success/failure. And for the most part, that's okay. If you only have one person with Stealth maxed out in your Pathfinder group, you really don't need a complex system just for their one skill. Stories revolving around stealth were cool, but it wasn't the mechanic that interested me, it was the story telling between me and the GM that made it cool.
Project: Dark has really cool stealth mechanics. Now I can play a stealth based game where the mechanics are interesting AND the group can tell a cool story. The cool thing is it plays well with 1 player, where it feels more like some of the aforementioned video games, or with 3-4 players, it feels more like a heist movie with specializations, it works well both ways.
I'm super excited for this game. He's planning to do adventures for the stretch goals and last I heard, possibly organizing them in series to help make an adventure path style product.