My initial reaction from last night, haven't been back on today.
Voice activation
Very hit and miss for me, particularly trying to turn the thing on from standby. Just seems to completely ignore me. When it works its a brilliant way to jump between things instantly, when it doesn't you just feel like a complete pillock.
Controller
Feels very familiar, however I'm not sure on the bumper buttons. I'd never heard anyone really complain about them, they seem more awkward to press now. The ridges on the control sticks seem deeper, and feel like they rub on your thumbs more. Not sure why they changed the names/icons of the start and back buttons. Overall still a nice controller and see what it is like long term.
UI
Feels very unfinished and missing the wow factor. I use Windows 8 at home and at work and quite like 8.1, on the Xbox though it just seems like it is missing a little sparkle. Lack of background for example, it's just all very unfinished. The ridiculous tiny gamerpics for anyone on the 360 is embarrassing.
Games
Loving FIFA as always, enjoying DR3 and getting BF4 Monday. Feels like a small step up at the moment but still noticeable, looking forward for whats to come.
Overall I like it, but it does feel like given 3-6 months more it would have been a lot better. That's not an issue for me really as I'm quite happy to see it evolve. Have had a few errors and crashes so far. Nothing that made the OS unstable but a few annoyances none the less.