Esfenvalerate has replaced the naturally occurring compound fenvalerate (to which it is almost identical) for use in the U.S....The only differences in the two products are the relative proportions of the four separate constituents (isomers). Esfenvalerate has become the preferred compound because it requires lower applications rates than fenvalerate, is less chronically toxic, [emphasis soilman's] and is a more powerful insecticide. [emphasis soilman's] The compound contains a much higher percentage of the one insecticidally active isomer (84% for esfenvalerate and 22% for fenvalerate).
So my opinion is: natural smatural. The advantage of natural is you can grow it yourself, you don't need a large factory, and so you can survive independent from the military-industrial system. But in this case, esfenvalerate is probably better as an insecticide, than its naturally-produced relative. Less toxic to people, more toxic to insects!
And yes, I've used it myself, and it works well. Not as well as diazinon, but well enough, and it's less toxic to the environment than diazinon. It breaks down in the environment similar to natural pyrethrin, just takes a bit longer.
Diazinon just murders everything by the way. It is eerie the way it kills insects.
Note that esfenvalerate is fairly safe for birds, but nasty for fish. So if you are worried about ecology, you don't want to use it where it gets into your soil, and then your soil runs off into waterways.
It is very toxic to bees, but it tends to repel them, they don't eat it, so there doesn't seem to be a major problem in this area -- unless of course you are depending on bees to pollinate your vine crop (melons, squash, whatever). In that case, do not use esfenvalerate just before the plants will bloom, and during the time they are blooming.
I find it interesting that bees know it is toxic, and avoid it when it is highly concentrated, but eat food containing it when it is present in low concentrations. I wonder if other insects act this way also -- in contrast to the way they act with Diazinon -- what i've seen with diazinon is plants coated with dead insects. With esfenvalerate, what i've seen is reduction in insect infestation.