I agree with Tango on international upgrades. I fly BOS-LHR a lot, usually AA, in discount coach paying $350-$850. A business class ticket costs over $4,000 RT. The upgrade is 50,000 miles, which works out to 7-8 cents/mile value.

The only time I beat that was for a BOS-SJC round trip on short notice with no Saturday night stay, which would have cost over $2,000 RT, for 25,000 miles. That's a much better value than a cheap coach ticket to South America for 40K or 60K miles (depending on the season).

I get operational upgrades to London about a quarter of the time, which is nice when it happens but not frequent enough to count on.

Phil's right on about non-airline products. Those tend to price out at about 1 cent/mile.

It all depends on what you want.