We have annual passes for both Disney and the Universal parks now. The Disney ones we purchased at Downtown Disney and they last for a year. The Universal ones we purchased online and they last for 2 years.
We only bought the Universal ones because there was an offer on, buy one year get one free and as we knew we would be going the following year it made sense to buy them. We have also had an unexpected bonus holiday over there which has made them even better value.
The Disney ones we purchased because of the bonus holiday in December and also knowing we were back in July, thereby getting two admission periods within the same year, this made them better value than buying our normal tickets.
Unless you get a buy one year get one free for Universal or intend to be visiting more than twice in a year I wouldn't buy them.