It depends on the purpose. The effects of probiotics depend on the specific strain and the circumstances in which they are used. Most do not establish themselves permanently in the gut: they act while you take them. There is good evidence for some specific uses, such as preventing antibiotic-associated diarrhoea with certain strains, and less evidence for general wellbeing.
Recommendations from the Ultra test provide guidance on the types of probiotics associated with your profile. Always choose with advice from a healthcare professional, particularly if you have a weakened immune system, are pregnant or have a serious illness.