We've started a Tanstack Start project at work for a green field project, and it is honestly a mixed bag. It took a lot of trial and error to get the whole project working - the server parts were working without an issue, but there was no client hydration. After some debugging, we have found that there was an error being thrown during data preload which took down the hydration process. No errors were emitted, and the docs weren't were helpful at all.

But once we got that out of the way, it is pretty nice to work with. The FE developer is very happy with how fast the project is and how easy it is to just peek into the BE portion and understand it. Server functions are just so nice to use and everything gels together really well. But it's still early days though. I would only recommend it if you are OK with parsing not-really-great documentation and maybe some relevant GitHub issues from time to time.