Wrong reason's that I have seen:
Bought a 200-800 ah bank for their high acceptance rates then fed them with a 100 amp alt and 40 amp shore charger.
Bought them for the "low maintenance" then ran the engine 35 hours a season. Even the worst wet cell on the worst charger won't boil off on 35 engine hours per year.
Bought them for low self discharge rates then left them sitting at 50% - 80% charged after each sail and killed them in three seasons.
Bought them for the claimed "longer life" then found out they were dead in 4 seasons anyway, when his previous wet cells lasted 7.
Bought them to replace wet cells that died a premature death then the AGM's proceeded to do the same because the owner refused to address his/her battery practices.
Bought them because they don't give of "gas fumes" when charging. (His previous bank had zero signs of corrosion and in four years had never even taken water. When I opened them up they were still full and load tested at nearly new. Gave that bank to a friend who got two more seasons out of them. The "new" fumeless AGM's died at year five.....)
#7 "The guy at West Marine said they were the best."
He then proceeded to fry two stock alternators and finally had to buy a fully gourmet charging system for another 1.3k. Previous wet bank on dumb regulated alt lasted six years and cost $300.00 vs. $1000.00 + $1300.00 alternator regulator installation. $2300.00 vs. $300.00.
I'll be surprised if he gets 6 years out of the Deka/WM AGM's (only on year two now)...
I see and work on boats left on moorings, it's Maine, and the only thing I can ascertain is that AGM's do not like to sit discharged as mooring boats often do. I have seen LOTS of expensive AGM banks die before 5 seasons. I also check date codes every time I am at my local battery distributor on the core pallets. The 6 year old AGM core is a rather rare up here while the 6 & 7 year old wets are not all that uncommon on the pallet. Most of the dead AGM date codes are between 4 & 5 years at Dave's shop.
Nearly every boater I know with AGM's has bought them for their ability to take a fast charge then fed them with a diminutive alt that took no advantage of the high acceptance (wrong reason #1). I have personally seen a 100 Ah Lifeline take 85 amps for more than just a few minutes. I have yet to see any boater truly take advantage of the actual acceptance rates on a large bank because you'd need a HUGE alt to do so and small sailboat AUX engines just can't do this.
I replaced four T105's on a Sabre 362 that had lasted 7 years with a 90 amp dumb regulated alt. Boat resided on a mooring with no solar or wind. The Lifeline bank cost over $1400.00 and was flat dead going into the spring of what was to be their fifth season. At the same time we upgraded the batts we did a full gourmet charging system with 150 amp alt, dual pulleys, MC-612 Balmar regulator, temp sensing etc. etc. on and on. He even bought a maintenance charger that was recommended by Lifeline tech support for the off season where they were stored in his 55 degree basement and cycled on and off the charger to keep them at 100% SOC. Total cumulative motor run time over the four previous seasons was just over 400. The bank had never been discharged below 60% SOC during these four seasons as monitored by Link 20. Dead, grave yard dead. Lifelines attitude, not out of the ordinary, try equalizing. He's back to 6V wet cells again, expensive experiment.. This is one I really feel terrible about because at the time I had bought the AGM static hook line and sinker and really pushed this guy towards this set up. I have wracked my brain as to what cased this but there is no answer. Everything was done by the book.
Oh and then their is my buddy who is the head systems tech at a very well respected boat yard here in Maine. He is an ABYC marine electrician, NEMA certified etc. etc. on and on and on. Probabaly one of the best marine guys I know in tems of knowledge. His own bank of AGM's in his own boat lasted three seasons (Deka's). He's gone to gel.
Don't get me wrong, there ARE benefits to them, and if you can truly take advantage of these benefits they can definitely be worth it. Most I see don't truly take advantage of the benefits. If you go AGM try and keep them fully charged as best you can. If on a mooring get solar or wind to augment. If you can afford a $1300.00 bank you should really take care of them. In my opinion and based on the "claims" these banks should have all easily gone 7+ years.
I had planned to go to AGM but I am still with wets because I have been able to live vicarously though many friends and customers AGM trials and tribulations.