I’ve been using XCode for over 2 years now, and had missed one important debugging feature (and my life has been more difficult for having missed it).
In the Breakpoints navigator, you can click on the + (add) button and select “Add Exception Breakpoint”. The dialog that appears will let you select specific breakpoints, classes of exceptions to break on, or let it break on all exceptions.
Without this I was guessing where the error occured. Luckily I was often correct, but what a waste of time.