Last week I decided to take the plunge and upgrade my household iPhone 4S's to iOS8. The rumors on the street have been mixed, but I usually don't pay much attention to rumors because they are very often wrong.
I am very happy I upgraded. All the fear-mongering about bad performance and bad battery life simply wasn't true for me. I'm quite confident that those folks just had a bad app that they didn't know how to update, or a heavily-used 3 year old battery. iOS 8 isn't going to fix your refrigerator, but it isn't going to break it either.
So what do I get?
First of all, I get a refined UI. iOS7 was a great upgrade, but now iOS8 is simply smoother all around. It erases all doubts I had moving from iOS6.
Family sharing is in place. If you have a household with more than one device, this is a big deal for making Dad's life easier. Or mine.
I get better battery management, with the incredibly useful Battery Usage feature. Unlike the similarly named Android feature, Battery Usage fully sums up the load of your apps on the battery and gives good advice on improving battery life. (The Android feature is just a modified top and has limited utility).
Pan-device searching with Spotlight is amped up. New keyboards. Notifications. Photo and Audio filters (VERY awesome, smoothly executed, and well behaved).
So far I have yet to have a single crash of either the OS or -any- application. ROCK SOLID.
Should you upgrade? You certainly don't have to. But I think you might want to.