As a developer and an end-user, I don't like anything that requires a plugin, and I feel that is the consensus among everyone but desktop developers.

Well written AJAX & Java just work better and don't require some annoying plugin, download, or upgrade.

And on a sidenote, if you're on a Mac pay attention to CPU and memory usage anytime the slightest amount of flash is being used on a page, it skyrockets, totally inefficient for no particular reason. It seems SLIGHTLY better in Windows but it's still a total resource hog.