Ross,
I had two West Marine AGM group 27 batteries that lasted 7 years and were installed by the previous owner. The charging system wasn't exactly well set up for them either, so I was pretty impressed by how long they lasted.
That being said, I'm currently 17 months into a cheap Interstate 6v golf car battery experiment that seems to be working out pretty well so far.
The 6v golf car batteries are about 70% cheaper and have a similar footprint to the group 27's that I replaced so installation wasn't too much of a problem. They did have to be wired in series though so there is a cost associated with some new battery cables. Also there is the routine watering of these batteries that I never had to think about with the AGMs. I've had to learn to store a gallon of distilled water in my ice box that lasted me about a year.
The golf cart batteries offer 210ah versus 172ah (I had group 27's) so there is some increase in capacity there, about 22% which has been nice for my typical use patterns of day sailing and one or two overnights at anchor a month.
The golf cart batteries do require water service though on a routine basis, I find that I check them monthly and add water about every 90 days or so. My boat is in a slip year round in a warm climate on shore power. Neglecting to water these batteries would quickly become an expensive problem, the AGM's were very nice in that I never did anything to them other than stare in amazement over how long they lasted.
For me, the cheap lead acid 6v golf car batteries appear to be a pretty inexpensive solution to my typical use patterns on my Catalina 30. Given their cost I figure I'm going to break even at about the two year point and if they don't last significantly longer than that I probably won't try again given the reliablility of the AGMs previously installed.
If they make it three or more years though... well... I might just be sold on them.
Take care, DanM.