Unsurprisingly Stylus is one of the preprocessors that people want to avoid the most (beating less by 1% in user satisfaction). The preprocessor is very apathetic to what style you use - it's the antithesis of a linter. Want to use a semicolon on this line, and not one on the next? Go for it! Not the best when you want to write something production worthy.
That said I've found it phenomenal for prototyping, simple codepen examples, and teaching css. I run the prototyping efforts at Atlassian and I've built a large amount of code prototyping tools focused on allowing designers to quickly build something out. The standard template I give to designers and students uses stylus by default because it allows them to make mistakes. People who are unfamiliar with code feel defeated when a small syntax error was the one thing holding them back - it makes them question their understanding of the intention of what they wrote, even when the meaning of what they had was correct. Stylus fixes so many of these things for me. I'd highly recommend doing any of your exploratory phases in Stylus.
That said I've found it phenomenal for prototyping, simple codepen examples, and teaching css. I run the prototyping efforts at Atlassian and I've built a large amount of code prototyping tools focused on allowing designers to quickly build something out. The standard template I give to designers and students uses stylus by default because it allows them to make mistakes. People who are unfamiliar with code feel defeated when a small syntax error was the one thing holding them back - it makes them question their understanding of the intention of what they wrote, even when the meaning of what they had was correct. Stylus fixes so many of these things for me. I'd highly recommend doing any of your exploratory phases in Stylus.