Unfortunately I saw the movie (which I wasn't that enthralled with) first; when I finally read the book I couldn't get John Cusack out of my mind's eye. He's great, but wrong for the character.
The book is very sharp and funny and gets you inside the protagonist's head in a way the movie can't and I loved it... until the last couple of chapters, when I felt it fizzled out. Hence three and a half stars instead of four. But definitely worth reading. I laughed out loud many times!