I just felt uncertain how long these things should last, so I bit the bullet and got two new Duracell AGM Group 65 batteries from Sams Club. Thought about Optima, DieHards, Interstate, and the like. Ultimately the AGMs for $159 including a 3 year direct replacement enticed me enough to opt for them.
Knowing how important ground wires are to computer circuitry, I had always kept my battery cables clean and protected with CRC spray cleaner and protectant. 9 years and 108,000 miles on a set of batteries to me is long enough. I read that others have gone longer, and I have a close friend with an '05, and he is still going. However, when I changed them out, I noticed that there is a lot more cranking speed back in the starter! It hits much quicker, and sounds as if it slows down in order to idle. There are other advantages as well! The alternator apparently does not get hit so hard to maintain voltage when the grid heaters cycle. When the intake grid heater cycles, headlights dim less, the wipers do not slow down as much, and the volt meter doesn't drop down near to the 12V line. It's a win win situation. Less load on the alternator, and better speed on the starter which will mean better life for the brushes and armature!
I have not had my old units tested yet, and currently have them in the garage hooked up to individual battery tender plus's so I can then have them tested to see if they are bad soon.