What are your thoughts on committing the .idea file? Does this make sense or are those custom settings for every user?

Official support says to commit the

.idea

folder partially: https://intellij-support.jetbrains.com/hc/en-us/articles/206544839

we don't commit IDE files and try to ensure our project can be developed and built without one

I don't commit it either. I really wish there were more documentation on what each IDE file does. Having a common setup for all the developers already in the repo would be so productive. Specially when someone joins a team.

We commit some files, most notably project codestyle file.

try to ensure our project can be developed and built without one

If you’re using Maven or Gradle you’ll still be able to build and develop the project outside Idea. Files in .idea mostly cover Idea-specific configuration (which gets synced from Gradle/Maven configuration) or Idea-specific functionality (and this doesn’t break other IDEs)

it probably would be okay as long as .idea folder only contains stuff that applies to the project in general and not individual developers

our codestyle is enforced by checkstyle plugin in the build

Checkstyle doesn’t format code automatically though. If some developer uses IJ, providing them with already configured code formatting settings is useful. Similarly launch configurations. Overall I don’t think it’s ever necessary to commit .idea files, it’s just convenient for people using IJ and doesn’t hurt those that use other IDEs