The word can strike fear into the hearts of gardeners. Like Quint's fingernails on chalk board in Jaws, I picture myself giving the speech...
"Y'all know me...y'know what I do... I'll get yer nematodes, but it'll cost ya..."
Even Spongbob had a run in with nematodes (not very accurately).
But as far as biomass goes, this is the biggest population on earth. The pile of nematodes weighs more than the pile of elephants, or blue whales, even (for the moment) more than the pile of people. There are more species here than you can shake a stick at; Jeff says nematologists have ID'd 20000 species, but expect there are a million! And like fungi or bacteria, only a few are pathogenic, the majority are benignly helpful, or actually beneficial. Predatory nematodes are known to most of us for grub control, but as Jeff points out, the mineralization that bacterial and fungal feeders do (with a whopping 100:1 C:N ratio, these guys move some nitrogen!) is really important.
But as Jeff also notes, this is one of the first orders to go in compacted soils. Can't move, can't eat. And compaction is a big problem in plowed land, chemically fertilized soils, or lawns where you drive in circles on a heavy tractor every week. Just sayin....