I opened up my MacBook and it informed me that XCode 5 was available and "would I like to update it?" Sure, why not? Well, I can tell you why not. XCode 5 made software written in previous version not work!
Yep, I learned that lesson with Xcode 3 and Xcode 4. (Or was it even worse, between Xcode 4.1 and 4.2 or something like that?)
Never update Xcode to a new version until you're ready to support a new version of iOS!
Newer versions of Xcode flat-out remove support for older iOS versions and Apple pulls the older SDKs from download. You can still find dubiously legal downloads of the older SDKs, but you can't target old devices.
Which is great, because originally the app I was working on was targeting iOS 5 on an iPad 1, and whatever the update was, it forced us to iOS 6. Oops - went to go to revert to an earlier version Xcode, and you can't! Because it's only available through the App Store, you can't get older versions anywhere. (And as said above, you can't just download the older SDK and use it with the new Xcode, because they pull those, too.)
So now I have a useless iPad sitting around my office that can't be used for anything. I'm pretty sure you're not even allowed to install apps that used to work on iOS 5 on it any more if they've ever been updated. It's just a useless hunk of no-longer supported hardware wasting desk space.
But, yeah, lesson learned: never update Xcode! Small patches (like a 0.0.x upgrade) are probably OK, but when the major or minor version changes, expect to be forced to drop devices or rewrite code.