As the title said,
I was considering using React in a production-ready project (in particular: Meteor's Material-ui module).
As it's not even at v1.0 then is it stable enough to use in production? What downsides are there to using a <1.0 version?
If you or the organisation you work for are a devoted follower of the semver method of versioning then it would be very risky to use anything that is less than v1.0 due to the following clause:
Major version zero (0.y.z) is for initial development. Anything may change at any time. The public API should not be considered stable.
All this means is that that a library changing from 0.x to 0.y (including 0.x to 1) may have a completely different api which could mean you need to do a lot of work to upgrade.