Tomorrow, iOS 8 comes as a free update for all iPhones 4S and newer, all iPads 2 and newer, and the 5th generation iPod touch. As always, my advice is wait and see how others do.
For most of us, it will be fine. But, you never know. Is your phone OK right now? So maybe wait and see what happens to battery life,
Internet connectivity, email and critical apps for the brave ones who "just do it" tomorrow. What do you lose by waiting until those reports are in, versus what could you lose if your perfectly working iPhone stopped working perfectly?
If you are traveling, it's always a good idea to wait until you are back at home. If your phone is one of the rare few that brick, and a few always do, would you rather be home, yelling at your local Apple Genius, or sleepless and phoneless in Seattle, or Austin, or Cleveland (God forbid.)
In law school, my torts professor tried to explain the idea of a "slight risk of grave danger," by pointing to a giant fish bowl full of jelly beans. "Here," he said, "I have a few thousand jelly beans. They are delicious. One of them is made of cyanide, and will kill you quickly. Who would like to try one jelly bean?"
Most people will get grape, or cherry, or lime, or that weird white one… but someone's gonna get the poison one. Back up your phone and be at home before you take that chance.