Are millipedes good or bad for the garden?

Question

Are millipedes considered good or bad for plants?

From wikipedia:

Millipedes are detritivores and slow moving. Most millipedes eat decaying leaves and other dead plant matter, moisturising the food with secretions and then scraping it in with its jaws. However, they can also be a minor garden pest, especially in greenhouses where they can cause severe damage to emergent seedlings. Signs of millipede damage include the stripping of the outer layers of a young plant stem and irregular damage to leaves and plant apices, the very top of a plant.

Answer 1

Although millipedes can be quite beneficial in the compost pile, in that they help to break down the contents, they also feed on vegetables and can destroy seedlings, as you point out; the few, small benefits they confer are far outweighed by the the harm they can cause, particularly when their numbers are high - see Millipedes eating my vegetable seedlings .

Answer 2

It would appear that they are "bad" in my greenhouse. Something was gobbling up my basil seedlings at night--leaves and some crowns--the lower leaves of my seedling tomatoes were also being decimated. I couldn't see any bugs when I shined the flashlight at night and dug around in the dirt. In the daylight, I found hundreds of tiny, 1/4-inch-long millipedes in the top 1/4 inch of dirt. There were no other bugs, grubs, or larvae to be seen. I killed as many as I could find. I look forward to seeing beneficial results!