Depending on your needs.
If you need to download programs for the App Store as quickly as possible, then Xcode 3.2 will still be the most efficient use of time.
If you get a lot of new things from new features like static analyzer and SCM updates (for example, git is integrated), and you can afford to go a little slower (crash, learn new design and layout), Xcode 4 is great.
I spend time studying Xcode 4 because I am delighted with new tools and don't have massive time / money. I see this as an investment in my developer skills.
It is stable enough for development, but not for production environments: my own personal projects are executed in Xcode 4, but at work we use Xcode 3.2 to develop iPhone applications for clients.
Luke
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