Without compositing?
I can’t see a way to use it, at least not effectively. Its implemented as a shader, no falloff controls per material, etc.
How to apply it to ambient term only, as in http://mentalraytips.blogspot.com/2008/11/joy-of-little-ambience.html?
Any tips are welcome.
IIRC Ambient Occlusion is already part of the image when rendering without compositing.
But from what shows the article, it seems that passes are composited on top of each other. So to replicate what the article says, you’ll have to use the compositor anyway.
You probably wouldn’t believe it, but in other render engines AO is quite usable for many shader effects even without compositing.
About it being part of the image, well yes it is. However it isn’t usable as it acts additively, which means that you need to adapt all the shaders or all the lights or both to even out the blown out picture.
There is an Ambient Occluson shader available for the materials. His name is pretty straight forward 
I noticed it, however I would love for some examples how people use it in conjunction with other shaders in the node tree. I can’t find a way to use it. Can’t add it, can’t mix it, not without counterbalancing the rest of shading before it.
I tried it and find it pretty useless. Can’t ramp or invert it, light, color, mask, texture, deform, displace, control (particles, fallof…) with it.
One global setting for distance? Cannot set it per object, shader, material…
Are there any known plans to make it at least bit more practical or are we stuck by working around, passing those limits by baking or OSL (no GPU)?
The AO shader node has only a few uses…
Example, connect this AO shader only and bake an AO map. It renders faster.
Further development is needed.
Yep, I know & agree, but ain’t no coder 
Wish that something like OSL GAO2 would be implemented in node shading network available for GPU.