We' tried SBG this year. Bought a book, followed it and now that the growing season is well underway, I don't have many positive things to say about it. Maybe this is better for northern gardeners with shorter growing seasons. i planted some of the same things traditionally as well as in the bales, and the bale plants seem stunted and anemic, while the traditionally planted plants are thriving.
Other issues:
Grubs... huge grubs that must have been "planted" by some beetle, which then attracted raccoons who tore apart the bales looking for an easy snack.
Birds and chipmunks pecking at the surface, digging holes which expose plant roots. I was hoping that being up off the ground would prevent this, but hey, they can climb... as well as earwigs and slugs, which have also been a problem.
Surface of bales are sinking in, disrupting roots, just doesn't seem like enough soil medium for roots to get hold.
Difficult to keep all parts of bale watered when using soaker hose.
The seeds planted on top of bale got washed away with the first heavy rain storm we had.
The upside:
We have some great partially decomposed nutrient enhanced mulch for the gardens that ARE thriving.
This experiment was a bummer. :/
We'll still try to grow some cool weather crops and see how they do into early Winter since we have some bales we haven't planted anything in yet.