I've been drinking kefir for over 20 years and drink a cup every morning. I don't have any gastro-intestinal issues, but then again, I was raised on the stuff. I also follow my daily kefir consumption with a good 10 cups of coffee and at least half dozen diet sodas, and nope, no problems.
Kefir grains double in size every 20 days or so, and many people either sell or give away their excess on Craigslist.
My husband resisted my suggestions to drink kefir for his chronic colitis for years, but finally gave in during an especially bad attack. His colitis was gone very next day, and he's been drinking it too ever since. He hasn't had an attack now in over 10 years.
I don't believe it's a magic "cure-all though" and what works for some, doesn't necessarily work for all. I feel it's worth a shot though.