I once did some minor patches on the OpenTTD code base, but didn't go far, as I didn't like the code style. (This is no objective critizism, I have an idea where they came from, but just not the style I at that time wanted to deal with for a fun/side project)

I assume the maintainers take pride in it, creating a game which is liked by many is rewarding. Especially if you like the game yourself.

Depending on your day job it can be quite relaxing to deal with other problems.

But if you don't like the genre and don't like the codebase and don't like the people in the community and don't like ... it's probably no fun :) in the end only you can judge for yourself.

One thing is certain: It won't earn you lots of money and it's unlikely to be the reference for a new job. (And therefore takes a work aspect out of it, which to me is good)

I had plenty of fun to make an AI for it. My first software project that was used by others. As bonus, it actually worked quite well (called AIAI - Automatic Idiot AI).

Though as a programmer newbie I got confused by race conditions introduced by fact that game may be saved at any time, potentially leaving things in the middle of construction.

I still have plans to get back to it at some point.

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AIs and many other mods were created independently, support for modding was beautiful.