I've done my share of Flash/Flex development. It's great: You can build very cool UIs and know they will run in the same way on all the browsers. There is a (mostly) sane component model to create your own widgets. And you use a (mostly) sane language to build the application. Unless you approach those libraries that are trying to Java-ize the Flex platform, but please keep yourself away of them for your own sanity (and productivity!).
The fact that Flex is open source is also a good thing. But the Flash Virtual Machine isn't. There the bad things begin.
Flash RIAs tend to break the web experience. It's not only they hiding the URLs. It's also about usability and accessibility.
“After just a cursory browsing, here are some of the usability and data accessibility issues we observed. You can't select, copy, or paste any text. Your browser's font override features won't work, so you can't adjust the font or its size to be more readable. Your browser's built-in in-page search won't work, and you can't use the keyboard to scroll through the text. You can't parse or scrape the data in any way; the design is fixed-width, so it's not going to work well on different screen sizes; and browser plugins, like Greasemonkey, can't adjust anything. Basically when it comes to text at all, if you don't like the style or are visually impaired, you're screwed”
—Adobe pushes Flash and PDF for open government, misses irony
Unless, of course, that Flex get fixed. I have no idea how, but the idealist in me says that open sourcing the VM would help a little bit...