All the brand touting aside... I REALLY like the new XBox system.
Other than the interface now being much more Wii like (and based around it being a family entertainment system, not just a gaming console) the best change so far is that small videos are now properly full-screen... which stops my wife messing up the aspect zoom on the TV 
The new avatars are fun and you can save multiple outfits (and for the ladies there is a top and pants or dress option with diamond rings and jewlery... only cork heels and flats though... no real formalwear options) and alter your avatar at any time, and take a picture as your gamer pic.
... I just can't get over how much it looks like the vista application switcher (with the flippy pages) with Wii avatars (they seem to have that Duplo plastic look).
I've yet to really look at the new Marketplace, Live and gaming stuff (I got booted out of the living room so that the media centre can be "tested" with Gosip Girl...)
As it took me less time to find stuff than it initially did on the old interface, and the videos resize to full screen (a basic but crucial fix) I say its an improvement.
























