We frequently visit Montreal, and for years I've been bringing home the spice blend used by Schwartz's for smoked beef. I buy it in bulk (Usually about 5 pounds) so there's no ingredients list, but I'm pretty sure they have saltpeter in it.
About two weeks ago I did an 18# chuck for our staff, and my wife asked that I use the Montreal seasoning. It's been years since I used it on something that big, so I seasoned it up, pretty liberally, and let it sit for about 3 hours before it went in the egg. That roast was the most tender I've ever done, and the smoke ring was to die for. A good inch and a half into the meat.... seriously.... My wife says it's the best I've ever done, although the source of the meat, and the cooking process was always the same.
I did a little research on saltpeter and learned that it was a tenderizer and preservative. It's actually potassium nitrate that turns into a nitrite, that isn't the best for us, but... we haven't stopped eating pastrami sausage, or hot dogs either. It's also used in cigarettes, ice cream and toothpaste for sensitive teeth. So, I was wondering why it's never mentioned here either alone, or as a rub ingredient.
BTW, according to everything I've read, the theory that it affects your sex life is an urban legend. confirming link