What an interesting, original story. I don't think I had an imaginary friend as a kid (I just read a lot of books), and I've never really thought about them much, but I kinda want to believe this is a true story of real imaginary friends. And it was as sweet and odd and funny and profound as you might think (or even more than that, maybe, if you're thinking "not very"). 
 The story follows the premise of the description: Budo is afraid to die, so he doesn't want Max to stop believing in him because that's how imaginary friends die. But what if the only way to save Max from a terrible fate is to sacrifice himself? 
 Budo himself is the perfect main character for this story. The things he doesn't understand about the world provide constant comic relief (the "damunion," haha). But the things he understands, with his childlike mind, he explains with clarity and insight that go right to the meat of the issue. 
 It's a very human story, despite its imaginary component. Nobody wants to die and most people fear death, so I think we can all relate to Budo. At the same time, we have to sometimes do things that are dangerous when they're the right thing to do; otherwise, there's really no point in living in the first place. I think that's Budo's ultimate lesson. 
 It's a very sweet story, told with a uniquely mature, though still childlike, voice. The writing maintained Budo's hybrid child/adult, world-straddling voice excellently. And the world of the imaginary friends was fascinating and introduced many characters I probably won't ever forget. 
 I think fans of Dicks' novel Something Missing would enjoy this one as well (and vice versa).