AGM batteries are designed as drop-in replacements for flooded batteries and while they can be charged at a higher rate, they can be charged safely using standard flooded-battery charging systems. They are SIGNIFICANTLY DIFFERENT in construction, performance, and lifespan vs. flooded batteries, and that is why they are worth the extra money. Rather than argue with people who for whatever reason want to bash AGM batteries, I offer some information:
http://www.pureenergysystems.com/PESWiki/Directory/Bedini_SG/files/Vonwentzel-battery.pdf