Factory battery in my 96 TBird lasted 7 years to 2003, the factory battery in my 03 LS finally died this past winter after 6 years.
I replaced the battery in the TBird with a DieHard Gold, and it was fine when I sold the car a couple years later. Still running with the Duralast in the LS. It's been fine so far... we'll see how this next winter goes. I'm a little curious to see how it works out, as I couldn't find the correct battery for the car (trunk mounted, vented, group 66) nearby for any reasonable price. I used a different, slightly smaller sized vented battery and made it work. It was like 40-50 CCA higher than required by my car, so I'm guessing it should be OK. Oh, and of course, I saved like $60 over the crazy Lincoln/Jaguar spec battery.