Conditional to your definition of "serious", I did:
https://github.com/64kramsystem/catacomb_ii-64k. I essentially don't do technical writing anymore (and I had the impression that this topic isn't generally considered interesting), however, my considerations are:
1. there are three levels of refactoring: removing the extensive (unbearable, to be honest) boilerplate that C2Rust introduces; converting the design from "C with Rust syntax" to safe Rust; convert the design from unidiomatic Rust to idiomatic
2. as another poster pointed out, for non-trivial projects, writing refactoring tooling is a must (to remove the C2Rust boilerplate), in order to perform step 1
3. design refactoring (step 3) difficulty depends on the source code design; the code I worked with was relatively hard to refactor, as it was old (school), in particular, lots of globals; the difficulty was caused by the typical freedoms that C gives and Rust doesn't (in other words, the very obvious design differences between C and Rust); somebody did a C to Rust port of (I think) Zstd, which is a modern codebase, and I think much easier to work with (also because of less, or possibly no, external dependencies)
4. regarding the code understanding, if one performs the translation in the three-steps mentioned in point 1, at the end of step 2, one has effectively a safe Rust codebase, "just" unidiomatic
5. in terms of quantity of changes (but not time spent), it's possible to perform the bulk of step 3 with rather local thinking (understanding), but of course, most of the time spent is on major design changes
6. beside a few steps, I was able to perform a conversion in self-contained steps, which is very good news for this type of work. Even better, it's possible (but that's a niche case) to port an SDL project by using at the same time the C library and the Rust one!
7. however, I can imagine projects like Wolfenstein 3d to be very hard to port, since it's hard to port memory allocators and similar
99. most important of all: just converting to Rust will quickly (even immediately) find bugs in the source; I've found approximately four bugs in the source code, including one by Carmack!
All in all, I find this tool great, but somebody needs to work on refactoring tools, and C2Rust's output must be improved in order to be found usable by the public.