Ambient occlusion: when to use it?

Sometimes a need a quick render (like a clay one) just to view the model.
Is AO the best way to do it?

Well that depends on what you like, no?

Personally for previews I just set up a few lamps.

AO in general is never the best way to render a model since it’s absolutely fake. But that depends on your definition of best. IMHO one should mainly view it as a GI-substitute in animations when you don’t have the time or hardware to render with real GI.

As you said, I should use other method for rendering.
But why my scene - with AO - takes the same amount of time for rendering when I turn it of and lit the scene with a sun lamp? Shouldn’t AO renders faster?
Another thing, why material such diffuse color doesn’t renders correctly?

Nah, AO doesn’t necessarily render faster. Especially for exterior scenes, plain pathtracing is pretty fast. It will be much faster for interiors though.

As for diffuse colors not rendering correctly, well, no idea what you mean. You’re probably doing something wrong. In any case you’ll have to provide more info than that if you expect a good answer.

AO does not really cast directional shadows so if you are showcasing a model on a turntable you might want to use it. If you are trying to simulate an environment you probably want some kind of directional light to cast shadows. In that case turn your AO power down quite a bit and use it as a fill light.