Here's a good summary of the studies and general ideas about probiotics for human babies.
http://www.ifm.net/industry/probiotics.htm
There are really two products - PRE biotics - that just encourage beneficial bacteria to grow, and contain things that just tend to encourage the specific bacteria. And PRO biotics, that are actually LIVE bacteria. Usually lactobacillus.
We just have to keep straight what these products really DO and what they don't do, and not just believe a lot of wild claims in ads. We also have to understand whether the product we are buying, what it really contains, not only what it MIGHT do, but can that form of it actually do that and how much of a difference it really can make.
You see probiotics are not medications, so they don't get any controls put on them or anyone checking whether the product really contains a form of something that can actually do anything, and if it did something, would that even be to an amount we could even notice. A lot of times companies try to sell us stuff that might have shown a tiny difference that either is within the 'error range'* or is 'clinically insignificant'. We have to be careful as this is a supplement and the supplement industry is 100% unregulated.
*The error range is say, your scale measures weight give or take 2 lbs(that is how accurate most human scales are). So if it says you weigh 100 lbs, you weigh anywhere from 98 to 102 lbs. Say, if a study finds a product causes 2 lbs of weight loss, that is within the error range of the scales they used, it may or may not mean anything at all.