One of the big design problems that all gel batteries have (AGM as well as straight gel-type) is that they are EXTREMELY sensitive to overcharging situations. Some manufacturers are reporting that battery life in gel batteries will be reduced by almost 60% simply by charging them .7 volts higher than recommended levels (generally 13.7 volts) and an 80% reduction in life with a 1.5 volt overcharge. This is not an issue with dry cell batteries and is just something to consider when selecting a battery for your particular situation.

Those who have a need for extended battery power (e.g. huge audio systems, 120v outlets, extended storage requirements, and/or harsh environmental conditions) will require a vastly different type of battery than the daily suburban driver in a moderate climate. That said, since battery designs and their corresponding advantages/disadvanges are both material and significant - it's more appropriate to say that a particular design is better for a certain type of application rather than simply saying "brand X sucks, brand Y rules".

My 2 cents...