Last year I knew someone who contracted salmonella poisoning. Part of the treatment included taking Cipro. After a week in the hospital and finishing her antibiotic prescription, she developed a C. diff infection to go along with the Salmonella that was still there. After another 2 rounds of taking multiple medications (including a new C. Diff medicine, Dificid) and then starting probiotics, she has been doing well. Another lady I know has a stubborn H. pylori infection. She has been taking multiple medications for a long time but still has some horrible symptoms. She started taking probiotics and, although she still has some symptoms, she is doing significantly better. This got me to start taking probiotics. I started taking it everyday for several weeks and now take it 1-3 times a week. That is what works for me. I know people who take it everyday. I favor the brand pictured, but I think important points are that it contain several different types of bacteria and that it is stored in a refrigerator. They are alive! Refrigeration helps to keep it that way.
In an effort not to reinvent the wheel, here is a link that has a lot of good information about probiotics: http://www.medicinenet.com/probiotics/article.htm