Sometimes people approach a tool like Rails with a set of expectations like “Rails is for AJAX”. Someone might helpfully suggest that they take a look at Rails because it makes AJAX very easy. What you don’t expect is for someone to come back in a few weeks, with the understanding that Rails is the appropriate framework for implementing a set of Rich component widgets ala Flex. With custom helpers for managing pages with hundreds of ids, and expectations that you are going to have custom slider widgets that manage the dynamic scrolling and insertion of elements in a UL. It just makes for some very heavy browsing, AJAX is a great thing, but its overuse is the reason why my browsers are feasting on memory.
I’ve seen it. Someone gets the idea that Rails is some sort of rich GUI framework, and then they are perplexed to find out that any web technology that requires 20 server callbacks to load a page just isn’t “scalable”. It’s unfortunate when it happens because it simply provides fuel for people who said it wasn’t going to work in the first place.
If you need richness, go with Flex. Flex isn’t rocket science, trying to create the same level of richness in AJAX is rocket science.