Both pros and cons are relatively minor for now; for all intents and purposes, it still feels like iOS 7. The major changes to iOS 8 are in the app APIs (for which no apps exist yet) and interaction with OS X (these features are only going live with Yosemite in late October).
You'll see a few tweaks and new features here and there, but nothing that completely transforms the experience, at least until apps are updated to take advantage of the new frameworks, which should be relatively fast.
As far as cons are concerned, iOS8 is significantly more buggy than the latest version of iOS7 is. Most people report improved performance on iPhone 5, but I've personally seen the opposite.
Battery should be more or less the same, as well.