I first started taking them about a year ago, but was not ready for them. I needed some healing in my gut and initially reacted to the type I was prescribed, with lots of gas and discomfort.
I had the type changed and did a little better, as my digestive system healed.
I now drink a concentrated smoothy for breakfast and it includes a probiotic (in powder form) that I originally had trouble with, but now have no issues. My digestive issues have improved tremendously, but I attribute only a portion of that to taking probiotics (though they play a strong supporting role).
Bottom line is, they are necessary to everyone's system. You can destroy them with a poor diet (and increase the bad bacteria at the same time). Re-introducing them into your system may take an adjustment period. It is a waste of money to buy probiotics if you continue to create poor living conditions for them with a bad diet. But with a good diet, they will do their job of breaking down your food so you access the nutrients better.
They are living creatures, so they have a very short shelf life unless refrigerated. There are several types, some products like yogurt contain some or all types (read the label). This may be enough for some people, but if you need to avoid dairy products, not always an option.
That's some probiotic basics. I hope it helps