A good updated summary on probiotics from 538 GutScienceWeek:
This link reviews a good deal of science and has a nice table explaining costs.
Take home message: Probiotics which vary greatly by strain and often lack rigorous production standards may be beneficial for specific conditions like preventing antibiotic-induced diarrhea but probably are not beneficial on an ongoing basis.
The final post in the series looks at How the Gut Affects Your Mood.
While the author explains that there is likely a microbiome effect on the central nervous system as well as some intriguing animal studies, it is too early to know that manipulation of the microbiome will have beneficial effects on neurological/developmental concerns.