I work for a company mostly using Java, but I choose to rewrite a preformance critical part of the system in Rust. Before this rewrite I had only written some smaller thing in Rust, so i was familiar with the language, but not very experienced.
When I did the re-write I expected the performance increase, I was also impressed by the concurrency and the safety around that. It is of course still possible to deadlock, but most other issues around concurrency just goes away. I found that rust really lives up to the credo "fearless concurrency". However, all of these things were expected.
One of the things I had a hard time adjusting to was all this Error handling that was required, and getting used to the ? operator, learning to use the thiserror crate. I initially thought that this was the weak point of Rust. After a while I learned to use it and realized that it was quite the opposite. When I learned how to do the error handling, I suddenly found that the result is that there are no surprise errors left in production.
The unexpected benefit I saw was that the logfiles have shrunk with a factor 100 from Java to Rust. In java there are so many things that might go wrong in so many ways, that you need to log the internal state all over the place to be able to understand why something blew up. In the rust version, I get a clean and handled error in my log stating exactly what went wrong. Being forced to handle errors, combined with great mechanisms to do it, is a under appreciated side of Rust.
What is your most surprising benefit of using Rust?
On a more serious note than my other comment, Rust has shown me how much difference having good tooling can make. In Rust adding some new dependency or producing a performance flame graph or checking all dependencies for licenses you don't want is completely trivial where in other languages you would take multiple hours to get things working (I am looking at you, C and C++ in particular but not only those).