The eggs are great. For instance I can be up and running building an SDL2 game or make a webserver quickly.
It has good emacs support with geiser-chicken.
Because it compiles down to C the FFI provides a lot of nice ergonomics beyond what I get with Chez Scheme.
Error messages are actually useful with a stacktrace. Can't overstate this.
It supports optional type definitions which reduces the number of tests I need.
The docs are adequate, though I frequently add "MIT scheme" to my search queries. I wasn't able to get chicken-doc setup in NixOS, and the docs website has gone down on me a bunch lately. I wish Chicken had complete documentation available including its eggs in an offline format, like a PDF.
What about Gambit?
Perhaps it's a better, optimizing compiler but the built-in library is rather minimalist?
The compiler. That’s the gem of the system.
The generated C is reasonably portable, so you can run Scheme programs on systems that “don’t run Scheme”.
What happens if you try to run Python eggs under Chicken? Do you get Roko's Basilisk? :-)
Pythons are too cold to hatch Chicken eggs, and viceversa
Self-contained, compact, compiled, fast, ready to use.
SBCL is not compact and its "image" concept is not universally liked by everyone.
Guile and Racket are not fast (nor compact).
Chez Scheme is not "ready to use".