In a prior boat I used the Kirkland deep cycle batts 3 years then sold the boat. Then I went with POWER-TECH AGM from Defender and they lasted about 5 years. My current set of Kirkland were purchased last summer so it's too early to judge.

Generally I'm very impressed with Costco quality, so I expect good things and if for some reason it doesn't work out I know 1) I spent less and 2) I have a nearby store who will stand behind it.

As for my use: The boat is kept on a mooring with enough solar to keep the batteries at 100% while keeping the beer cold. In the early dawn or on a cloudy day the batteries might go down to 90% but by the end of each day the Xantrex XBM reads 100%. The only time it goes down further is when we cruise and open the fridge more, use more lights, instruments, autopilot etc. I never let it go below 50% or 11.9v without running the engine or shutting things down. (I also have a low-voltage disconnect on the fridge, set for 11.8v). I'd rather have warm beer and a few spoiled food items than fried batteries worth hundred$.