So I've been on the Galaxy Nexus since release date, and just picked up the One. My favorite feature by far is the speaker. And not just necessarily using them for music. I do that too, but the biggest upgrade is when I use Hangouts. I have a new baby, and my parents live across the country, so we use it 2-3 times a week so they can see him.
So many times I've used hangouts and couldn't hear a word the other person was saying if there was any background noise at mine or their end. Now these speakers are just amazing. They're so loud and crisp.
Besides the speakers, I love the screen and the build quality. It feels like I'm holding an actual nice piece of technology.
One thing I don't like is Sense. Coming from a Nexus, it's just so weird. I may start using a launcher. But I think it'll just taking some getting used to.
Love the phone overall though. Glad it came out at the right time...