"If you think the manufacturer's sticker gave you a reliable
estimate of the city mileage for that new car of yours, you should
try negotiating traffic on the hills of San Francisco. You'll
discover pretty quickly that in my town, the old adage 'Your
mileage may vary' has never been truer.
"Likewise TCO (total cost of ownership) estimates for enterprise
software. Commercial vendors such as Microsoft continue to use TCO
as one of the biggest hammers wielded against Linux and open
source, despite the fact that it's virtually impossible for a
single TCO estimate to match every possible usage case..."