Connecting to headless/API CMSs?

I'm fascinated by services that provide a API (Headless) CMS. Here are some examples:

https://prismic.io/
http://osmek.com/
https://www.contentful.com/
https://cosmicjs.com
http://getdirectus.com/

Has anyone had experience using Grav as the front-end and one of these headless CMS as the content/backend?

If you remove the theme (Twig) layer from Grav, you can call Grav a headless CMS too. Instead of showing nice pages, Grav would output plain JSON.
We're going to make this possible, too, in the future, with the integrated API. (and it's already doable with a plugin, without waiting for this to happen)

So to me (personal opinion), it doesn't sound like a thing that's worth diving into, especially seeing many of those are proprietary / hosted (black box) solutions, so less flexible than Grav, and you have little control over those..

I would use such headless CMS only as the "backend" of a JS app, not a user-facing website which is where Grav shines, and already provides all the mechanics to do so.

I understand what you are saying. Even so, I am seeking a frontend to a headless CMS product like the ones above ( also I'd include www.moltin.com ). They do custom content types (like Drupal does) very well, and I'm not seeing the same functionality in Grav. I've been considering using things like Grav, Jekyll, Kirby for the frontend theme/static pages, and for the dynamic content to use a dedicated headless/API CMS like the ones I mentioned. I really like the idea of dividing the project into two modular halves.

Although on Grav homepage:

"Dynamic Content Types
The flat-file nature of Grav lets you define custom fields for any of your pages, including modular content"

I do wonder if a standalone service like moltin.com or prismic.io is better able to handle large amounts of site data, like products, events, etc.

Grav does provide the ability to have custom content types.

You can have your theme provide custom page templates, and when creating a page through Admin, you choose what's the template of a page.

In Admin you can also set a blueprint for a page, which will create a different UI for different page types, see https://learn.getgrav.org/forms/blueprints#pages for more info on that.

If you look through the Grav Skeletons, which are basically demo sites, you can see what I mean with the difference between RTFM Skeleton, Blog Site Skeleton for example.

They all provide custom page templates (blog, blog post, documentation page..) and they are just pages.

OK, I'll check those out. Thank you.