So far, so good. iOS 5 appears stable on my iPad and iPhone. There aren't loads of brilliant new features -- for that I'm thinking I'd need Siri on the iPhone 4S -- but there are lots of small but useful changes that make it worth the hour or so to upgrade. My top new features so far are:
* keyboard shortcuts
* split keyboard possible (on iPads)
* dictionary definitions
* 4- or 5- finger swipe up to get to multi-tasking list
* 4- or 5- finger swipe left or right to move between most recently-used apps
And this is without yet trying the iCloud features. More on that experience anon.
For a much fuller list see http://www.macstories.net/tutorials/ios-5-tips-tricks-hidden-features/