I made these small test boxes, on which one can test and compare easily volume light
An effect that is in the beta versions of blender now.
I wish to share it so here is my testlab
https://dl.dropboxusercontent.com/u/54767531/light%20test.blend
So far i noticed that i might revert from cycles based emision planes in various scenario’s.
They always looked nice and gave often natural lighting, but they are not beam lights like suns and spots.
From i above you might notice i didnt include hemi, but its effect isnt working i think (or not in current beta’s ?).
Also i note that setting Clamp settings kills the effect mostly 
And that its often quite rich in pixel noise, giving it often a pixelated look.
I would wish for a layer output of the volume shaders, so one could put a blur over it to reduce pixelation a bit.
Because as it seams now, large renders takes many hours even on my octo core, and i asume GPU’s too.
Sometimes i see render preview errors, as in the test box above the vertical sun rays are not always visible at each box roof hole, but overall it is a nice adition to it.
Recently i used a volume shader to better create dark areas (instead of the light beams); i mixed a drawing without volume shader, and one with; then used both images to find an indeal middle in composite mode…
well sure there is more to play now
