My Oft-Repeated Advice -- Hold off on iOS8
There is nothing in the new Apple OS update that is particularly pressing, and even if there were, don't update on the first day. Wait. I gave this advice to my family for iOS7 and saved everyone a world of grief. One would think that Apple would have a way easier time with releases than, say, Android or Windows. Apple OS runs only on Apple devices, whereas Google and Microsoft have to deal with all sorts of hardware compatibility issues. Nevertheless, Apple has had many issues with its round-number OS releases such that there is no reason to rush. I suggest you wait 2 weeks, then Google "iOS8 issues" and "iOS problems" and see what you find. If nothing scares you, then update.
Here is the problem with Apple - whether it be OSX or iOS or even iTunes - it is almost impossible to roll back. I hated Windows Vista and Windows 8 (Windows is sort of like original-series Star Trek movies where every other release sucks more than average) but I was able to roll back in both cases. Short of rooting your iPhone, I am not sure iOS rollbacks are even possible.
