I ran across a nice intro article to using assertions in iOS Working with Assertions to Debug your Apps. It describes the macros NSAssert, NSAssert1, NSAssert2, etc. and how to use them. One item mentioned in that article is different in XCode now – the macro NS_BLOCK_ASSERTIONS is defined by default for Release builds.
The article started me thinking about how one might report crashes and user-generated assertions in a production app. I came across the article Handling crashes and NSAssert gracefully in a Production App. Not only does it talk about how to continue using assertions in a Release build by subclassing NSAssertionHandler, but it also makes reference to using a service such as Flurry or Google Analytics to log them as well.
Web searches showed that Flurry and Google Analytics are both nice choices for getting app metrics and logging errors/exceptions. I spent some time looking through the Flurry web site and documentation and was impressed with what I saw. I found a nice article that talks about Flurry and includes a video on how to get started with it – Find errors in your iOS app before your users do!