As a Java developer who has over the last couple of years taken a tour of the different RIA frameworks, I find Silverlight fantastic.
Flash, if you're a designer is awesome. But personally, as a developer, I found the environment pretty basic compared to server side java. Flex Builder is kind of crappy(at least as of 6 months ago), lacking many features even free Eclipse-based tools provide (such as meaningful refactoring). And the programming api outside the gui toolkit is only skin deep. And, which really makes me sad, no eval() in the runtime.
Love Microsoft or hate it, Moonlight/Silverlight is something devs should be excited about.
I also hope Adobe learns from it for future versions of Flex/Flash, continuing to drive innovation.