I've downloaded it on my iPhone and iPad now and got to grips with it for a few days now.
What I like:
- Quick reply to messages is really nice, and it will be a hell of a lot nicer once WhatsApp and the like integrate it.
- Having the option to install different keyboards is nice. (should have been an option years ago)
- Enhancements to the App Store and iTunes are actually useful, e.g. trending section, bundles.
- Quick Type is pretty nifty.
- Photos.app and photo editing have some really nice enhancements.
- Multiple mail drafts and gestures are really nice.
- Safari extensions and enhancements are great.
What I dislike:
- Extensions are not all that useful so far. (not really iOS 8's fault, but it looks like we will have to wait until extensions actually become useful and look nice)
- Continuity and Handoff between iOS and OS X are not yet available. I was really looking forward to this and I see it as one of the main strengths of iOS 8.
- iCloud Drive not yet available.(see above ^)
- It's a buggy release in general.
- The install process. 5.7GB required is no joke when you have 16GB devices.
- "Hey Siri" should work when the phone isn't plugged in and has above 20% battery.