Who or what is IMO? I can't tell you why they like it, I can only tell you why I like it.
It remains my best model of how the brain works, he explains this by using an anthill as an example.
Then there is the palindrome dialog called Crab Canon, 2 pages of dialog which reads the same forwards and BACKWARDS.
This book is full of information presented in a very creative fashion. I Would recomend reading it slowly, digesting each chapter before starting the next. If you have intrest in strange loops, this book is required reading.
It is a challenging intellectual exercise to read GEB. Some of his points are very interesting, and it presents Godel's theorem so that it is relatively easy to understand the proof as opposed to just the theorem.
Sometimes the diagogues between tortoise and achilles seem a bit pointless, but then something clicks and you can see the structure and the point the author is making.
It is fascinating how the concepts of the strange loop as presented in Music, Math and Art are blended together in a work which becomes an example of the strange loop in litureature. This is all about self referential statements in many different forms.