I've been reading about WP API and I love the idea however is it far enough along to use? For instance I want to build a custom react front end but use the WP API. My concern is that I won't have access to certain plugin information. Is it possible to hook in and create a custom end point for plugins that do not support WP API?
That is indeed possible. You can for example add values to the response, add support for custom post types or add fully custom endpoints.
Please note that v1 will be replaced by v2 of the API. v2 is still a work in progress - has been for a while - and will introduce a lot of breaking changes, so it would be better to use the latest beta version of the API.