Last year I worked on a UK mobile phone and mobile phone deals comparison site: Omio.com
When the project kicked off, we knew we wanted a beautiful interface with rich behavior. For example, the ability to filter deals and compare phones needed to have almost rich client usability and responsiveness. At the time, the team believed HTML & jQuery were the best option for the entire website. Since this was a public facing site we also (obviously) had search engine indexing requirements. The site turned out beautifully in the end, but these days I wonder if strictly HTML & jQuery were the best solution.
The last few weeks I've been working heavily with Adobe Flex 3. I've been very pleased by how productive the framework has helped me become. Very often a stakeholder will ask for a feature that sounds hard to deliver, but is made trivial by some built in component in Flex. I went into the project interested in Flex, and after a few weeks I find myself completely on-board.
Flex gives me all kinds of components and effects. It relieves cross browser pain. It gives me views that can contain objects, not just strings. It gives me those things and many other things that I had considered before I gave it a shot. I wasn't convinced. On paper Flex had nice things, but I was missing the killer feature.
Now that I've used it, I've found what the killer feature is for me--productivity. There are plenty of things to complain about (just like every other technology), but the combined result of all the improvements is better software, faster.
Getting back to Omio, I certainly don't believe that the entire site should be done in Flex. However, the very dynamic aspects of the site would have been easier to develop had we used Flex. Off the top of my head I would surely have built the filters and the comparison logic in Flex. The phone and deal detail pages should be done in HTML because they are easy, static, and can be indexed; however, the dynamic aspects of the site do nothing for, if not hurt page rank.
There are valid concerns with RIA. Neal Ford recently posted a good blog entry explaining how choosing Flex or Silverlight can lock you to a vendor. This usually isn't an issue since you are often locked to a vendor anyway. However, on my current project we experienced a moment where we thought that Flex simply wasn't going to do what we needed, and we were going to have to start over from scratch. It turned out to be a false alarm, but Neal's post really hit home for me those few days.
There are benefits, and concerns. However, if you go into the situation with your eyes open, I think RIA is the right choice far more often most of us previously thought.