Windows Phone 7 sneaked up on the world today, but having played with it, I'll tell you Microsoft is putting all it's muscle behind this. No matter who you root for, to be anything short of impressed is stupid.
How does it feel? Nothing like an iPhone, for starters. The slippery rotate-y screens may take a little getting used to, but they feel right. Microsoft deliberately wanted to get away from icons and this notion that all behaviours get the same size button on the home screen, and you definitely get more of a sense of priorities here: Entertainment, social networking, photo sharing - those matter, and oh yeah, here's a phone if you need a call, and here's a browser if you need that too.
It's hard to tell from looking at this stuff, but much of it is customisable, including almost everything on that home screen. Don't let the uniformity of design language fool you, there will be a lot you can do to differentiate from other people.
As you can see, the fluidity of the "panorama" navigation is here - when you enter a hub, you get those little teasers to the right, showing you want you'll get if you flip one screen over.
Though details are scarce in these early days, the device here is built "to spec", so probably running 1GHz Qualcomm Snapdragon processor. I can tell you that everything ran smoothly. This is obviously too early to make any technical statements, but it really was impressive, and where there are a few hiccups, it's hard to say whether it was human error or a glitch, but we'll leave it be for now. This is just demo software.
As you can see from the screenshots above, most hubs are fleshed out, though we couldn't have a look at Marketplace. Some of the shots here are "in between" shots, that moment between tapping a start screen element and the whole hub springing in behind it. There is also one shot of the slide transition from sleep screen - which has a lot of great heads-up information - to the start screen.
There aren't a ton of answers yet, but what we do know you can find above or in Matt's piece: Windows Phone 7: Everything Is Different Now