I use S7 in my open source Scheme-for-Max project. I have built a Max/MSP external enabling one to script and live code Max/MSP (a computer music platform) in Scheme. I love it, I checked out many options before settling on S7 and have been super happy with the choice (guile, gambit, chez, chicken, chibi, racket, ecl). It's in many ways similar to Janet and Clojure: a single-namespace lisp 1 but with keywords, CL style macros with gensym, and environments. It is also close to Guile, but is liberally licensed.
The best part of S7 in my opinion is how easy to embed and extend it is. The C FFI is really easy to use, and all of S7 fits s7.c and s7.h, making building very easy. You can see some S7 in action in Max/MSP here: https://www.youtube.com/watch?v=ErirIFCTdjg&t=2s&ab_channel=...
It will be seeing new releases this semester as it's now my Masters in Music Tech thesis project at the University of Victoria, working with Dr Andrew Schloss. I plan to have full support for running the Common Music algorithmic composition toolkit in Max/MSP this year.
I have also used S7 in a Juce applications, which is what the normal version of Common Music does in its "Grace" host and Christos Vagias has been using it in iPlug 2 for VST plugin development.
The best part of S7 in my opinion is how easy to embed and extend it is. The C FFI is really easy to use, and all of S7 fits s7.c and s7.h, making building very easy. You can see some S7 in action in Max/MSP here: https://www.youtube.com/watch?v=ErirIFCTdjg&t=2s&ab_channel=...