I like JSON5 and have used it some. When GPT was younger and I was parsing its JSON output directly, JSON5 was forgiving in useful ways.

The one thing I really wish it had was some form of multi-line string, mostly so I could use it with line diffs. Also sometimes for really simple property editors it's nice to put the serialization in a textarea, and that works fine for everything but multiline strings.

(I suppose you can escape newline characters to put a string on multiple lines, but I find that rather inhumane and fragile)