ERF is the story of four primordial friends taking on the danger and adventure of the prehistoric world. It's illustrated by Rob Steen and it looks great: the characters are colorful, wide-eyed, and absolutely not the sort of thing you'd expect from a Garth Ennis-penned book. Backing the project can get you anything from a thank you in the book to a signed print to a t-shirt.
There's no question that Ennis can write. Regardless of your opinion on the vulgarity of his work, the stories are tightly plotted, the characters are fully realized, and it's always entertaining. There's no reason to think ERF book will be any different, but if nothing else it will be interesting to see how Ennis' trademark style works with a wholly different audience.