Not so long ago, whenever people in the IT industry talked about Microsoft's rivals, the one name that would crop up was Sun. Ever since Microsoft introduced the .NET framework, the ongoing debate was always - Is .NET Framework better than J2EE? And developers would often take sides, depending on which camp they were in.
Now, a few years down the line, with the launch of Silverlight, the spotlight seems to have shifted to Adobe in terms of rivalry. Although Sun does have some products here and there in the field of rich Internet applications (RIA) they don't seem powerful enough to cause Microsoft to sit up and take notice. Not so with Adobe. Adobe's Flex is said to be one of the biggest competitors to Silverlight and the current ongoing debate seems to be "Is Silverlight better than Adobe Flex?" Again, you will find developers taking sides depending on which camp they are in ;-)
I don't know much about Adobe Flex so am really not in a position to say whether it is better or not.
Personally speaking, I am quite enthused about Silverlight and each day that I discover something new about it brings a new high.