Absolutely terrible, in every way imaginable.
The Unfunnies from "Kick-Ass" and "Civil War" writer Mark Millar lives up to it's name. Expecting to laugh while reading this comic is like expecting to leave Las Vegas with all of your money AND your innocence intact. The content of the book is purposely disturbing and shocking, including scenes where adorable cartoon characters are killed in gruesome and horrible way, which is all very disturbing... if you've never seen South Park, Happy Tree Friends, Sealeab, Harvey Birdman, or any of the other hundreds of shows that have all played this exact concept for comedy.
What separates this comic from all of those other things is that The Unfunnies wants us to take this concepts completely seriously. This is Mark Millar's attempt at a serious, straight-faced horror comic. And boy, does it fail at that.
Riddled with plot holes, lackluster art, terrible photoshop effects, and general crassness in place of plot or characterization, The Unfunnies is a thoroughly unpleasant experience. Those who like funny animal books will be horrified, and those who like shocking, thrilling horror comics will be bored out of their skulls. This is a comic that believes itself to be shocking and terrifying, but is only crass and boring.
The "shocking twist" of the comic, that Troy Hicks has replaced himself with a character in his own book and is slowly corrupting it, isn't that much of a twist, considering that the book practically tells the reader exactly that in the very first issues.
To the book's credit, the concept is wonderful, interesting, and boundlessly original. Unfortunately, like most of Mark Millar's body of work, the great concept is wasted on poor execution.
If you're looking for a funny comic, stay away. If you're looking for an interesting horror comic, stay away. If you're a Millar fan, you're the only person this book will truly appeal to.
A few people may enjoy it, good for them. If you can gain enjoyment from this book, more power to you. I certainly couldn't.