Several randomized, controlled trials have reported on the use of probiotics to treat a wide variety of disorders, including constipation, colic, irritable bowel syndrome, inflammatory bowel diseases (Crohn's disease and ulcerative colitis), allergies (atopy and eczema), Helicobacter pylori (the bug that can cause stomach ulcers) colonization and eradication, antibiotic related diarrhea, urinary
tract infections in preterm infants, and others. However more studies are needed before firm recommendations can be made, especially for infants and children.  One problem is that the forms and types of probiotics used varies tremendously.  Production is not as regulated as drugs that are monitored by the FDA.  Additionally it is known that if one stops using a probiotic after 'loading' their intestine with these organisms, the intestine quickly reverts back to its basal state (with whatever organisms it is programmed to hold).

However the question of whether this is a 'fad' is easier to handle.  In the future, once we have more information including studies underway now to study the intestinal bacteria (the 'microbiome'), we may be able to use taylored probiotics for specific disorders.  Thus, while the evidence to use probiotics at this time is minimal, the field involving probiotics (and prebiotics) is likely in its early stages and will likely open further exciting areas for future research and treatment strategies.