Microbes are not all created equal. There are different levels of symbiotic relationships here; some help the plant indirectly because they compete with pathogens and do not hurt the plant. Others will break down dead plant matter in to useable nutrients. Some help hold the soil together, others help move nutrients in the soil.

The stuff I use gives me great results and is worth the money to me. I use this stuff from fungi.com which contains bacteria:

Beneficial Bacteria Bacillus subtillus, Bacillus licheniformis, Bacillus azotoformans, Bacillus megaterium, Bacillus coagulans, Bacillus pumlis, Bacillus thuringiensis, Bacillus stearothermiphilis, Paenibacillus polymyxa, Paenibacillus durum, Paenibacillus florescence, Paenibacillus gordonae, Azotobacter polymyxa, Azotobacter chroococcum, Sacchromyces cervisiae, Streptomyces griseues, Streptomyces lydicus, Pseudomonas aureofaceans, Deinococcus erythromyxa

and fungus:

Endomycorrhizal fungi
Glomus intraradices, Glomus mosseae, Glomus aggregatum, Glomus clarum, Glomus deserticola, Glomus etunicatum, Gigaspora margarita, Gigaspora brasilianum, Gigaspora monosporum


Ectomycorrhizal fungi Rhizopogon villosullus, Rhizopogon luteolus, Rhizopogon amylopogon, Rhizopogon fulvigleba, Pisolithus tinctorius, Laccaria bicolor, Laccaria laccata, Scleroderma cepa, Scleroderma citrinum, Suillus granulatas, Suillus punctatapies
Trichoderma Trichoderma harzianum, Trichoderma konigii