This article provides a pretty good overview of probiotic issues. A couple of good snippets:
In most situations, probiotic administration results in an increase in fecal counts of bifidobacteria and lactobacilli, a decrease in fecal pH, and a decline in those bacterial enzyme activities that are associated with the development of colon cancer.
Bifidobacterium longum administration (as a pharmaceutical) resulted in higher fecal bifidobacterial and lower clostridial counts, lower fecal pH, and lower fecal ammonia concentrations.
They should provide similar benefits for everyone, but how much you notice it will probably depend on the condition of your gut flora. If you’re missing a lot of bacteria, the impact might be more pronounced.
It’s also important to take a high dosage. There seems to be a threshold level in most cases, which could be in the hundreds of billions. More strains also offer more benefits. ThreeLac is like a drain with dollar bills running down it. It has three strains and about as many CFUs.