No, they basically don't exist. I'm United Mileage Plus Silver and have never seen an upgrade (although now I get 'free' Economy Plus which just adds a few inches of legroom to the terrible experience). I've had friends put in for first class upgrades for me a number of times and never once seen one.
A "normal" person who doesn't rack up tens of thousands of miles a year will absolutely never see an upgrade.
It Depends. I will say that I get upgraded to first class on domestic flights all the time, but have only gotten two or three international upgrades.
In Delta's case, at least, it depends on how many seats are free, and how you rank. The ranking is based on all kinds of stuff. So for less-flown and less-upgraded routes, it's easy to upgrade. Other routes have more people flying first class by paying and more competition for the free upgrades.
