As regular readers will know, I have long favored using VG at no more than 20%, because of the real potential for thermal decomposition.
As an aside, I have noticed recently that - and this is a tentative conclusion only - is that my observation of more likelihood of burnt tastes with high VG liquids seems only to occur when there is ethanol in the liquid (as flavoring solvent). It is possible that the ethanol itself is the cause of the bad taste or that it affects the homogeneity of the liquid when wicking or in some other way is detrimental to the whole. This will take more time to elucidate. But I have not noticed in the absence of ethanol, any particular increase in bad tastes with VG at 50% which i recently retried, so the decompostion issue at that strength might be pretty minimal (even close to zero). With the caveat that by making the liquid more viscous and thereby less quick to replenish liquid at the heater coil, burning of the filler is still more likely. This can be mitigated by topping up sooner and vaping less vigorously.
Now the key point; and it again leads me to favor low use of VG. It relates to residual film, phlegm creation/thickening and atomizer/cartomizer cleanliness.
PG is known as an antimicrobial, and this includes yeast and moulds (probably the most serious infecting agents, over bacteria, as mycotoxins can be extremely toxic).
VG on the other hand, when diluted (as would be the case after vaping and the VG is in the mouth, throat and lungs), is a good medium for bacteria, mould and yeast growth (see for example candida infection symptoms). So in this respect PG and VG are very different - polar opposites, with VG being suspect as a good choice for vaping.
See, for example :
http://en.wikipedia.org/wiki/Growth_medium
This is acerbated by another difference - in the unheated evaporation rate. The difference is huge; and easily tested by anyone. Place a drop of VG on a saucer, and a drop of PG (or PG based liquid) on another. The PG will be gone in hours (as would a water drop) whereas the VG will still be there after weeks. The unknown, afaik, is whether the VG gets absorbed by the lungs or rather how well, or does it linger for a long time, coating the airways and impairing breathing, and of course providing a medium for microbials to flourish?