I've used the stuff for years and won't garden without it. It's a natural fungus (so, totally organic) which is added to the planting hole so that the plant roots directly touch it before you fill the hole. The fungus encourages lots of small feeder roots to develop, which overall boosts plant health and size. I can always tell which plants I've missed adding a small handful of the brown powder when I planted them as they're not nearly as large. You can never overdo the use of mycorrhizal as it's not a chemical or raw form of anything.
And yes, it is pricey stuff so I always scout all my local garden centres in fall for their clearances on it. Theoretically the fungus dies at the end of that year, but I've never found it to be so. I have a big container that's 3 years old and it's still giving me great plants. There are different forms of the fungus as well, ones dedicated to bulbs, to veg, to shrubs and to flowers. They vary slightly in particle size and fungus, but in a pinch I've used any of them for other purposes and they work.
