Hi I grew up drinking cheap and plentiful kefir, as I come from EE. It has live bacteria that eat up the milk sugars and leave the goodness. Kefir is traditionally believed to regulate digestion, slack thirst and make skin healthy. The modern pro-biotic articles I read are much along the same lines.
On the inverse, drinking too much kefir is not recommended (due to the same magic impact on digestion, lol!). That coming from a country where we, kids, would get half a cup of full-fat sour cream for an afternoon snack and have quark for breakfast and sent to drink raw milk in the summer to the country side, lol.
As a grown-up, I figure if kefir was good enough to rear the fierce Mongolian warriors, it's good enough for me
One thing to know about kefir, is that it is the best home-made, and that it is quite acidic. My husband and daughter not having been raised on it (and million other milk products) don't like it. We drunk kefir with a heaping tablsepoon of sugar or mixed with home-made jam (which was not pectin but sugar based, lol!) Unless we used it in summer soups with fresh veggies (tomatoes, cucs, radishes and dill)
Because of this acidity, I now don't really drink kefir (since nowadays throwing a tablespoon of sugar onto anything just feels wrong), but make quark out of it to mix with fruit.
If you are dairy intolerant, you can try water kefir, that is a similar bacteria that grows feeding on sugar and minerals.