My impression of the Start Screen is that it feels more like an addition than the main interface of the OS because I don't find myself returning to it on purpose. It feels sort of like a secondary UI compared to the desktop app. If we could take statistics on my usage of the desktop icons vs. Start Screen icons I'd say they are on par, but I'm usually just looking to see if any of the apps actually work in that screen or going to the Windows Store hoping that I see updates. With the desktop icons, I have an intention to launch something rather than wandering. I can most definitely blame this on my familiarity factor and the fact I have a mouse. Take away the mouse and I will be compelled to use the Start Screen. I think that is an important observation when you see Windows 8 on tablets.
I am skeptical of the Start Screen's viability in being the main interface for touch enabled laptops/desktops. It looks nice, but that's about it. Most of my work is done in the desktop app. Maybe when I start devving Windows 8 apps, then I might find the Start screen more useful, but until then there is quite a way to go. It is definitely targeted to tablets and it will be useful to that market segment, but until then most of the market will be non-touch laptops and desktops.
The Search interface is one of the more useful things that I use daily. Definitely an improvement over the start menu's search interface. You can filter by three things: Apps, Settings, Files. It is apparent that I am either looking for one of those when I go to the search screen, so kudos to the people who took part in making that decision.
Windows 8 is more than the Start Screen anyway, which most complaints seem to be geared at except for the exceptional few mainly game developers like Valve and Notch, who have valid concerns in what Windows 8 might be. I may be a hit or miss considering that XBox live's game publishing is so successful. I would contend to notion that Windows 8 is going the wrong way with the Windows Store as it is more of a consolidation effort rather than an Apple style power grab. You can still install what you want, so I'd like to think metro apps will be runnable regardless of being on the marketplace, but we will see very soon.
Besides the marketplace, Windows 8 is portable, can be refreshed while keeping all of your files, super fast screen rotation, supports all of your old applications, etc. Windows 8 is MS's re-initiation with policy changes, re-branding, bringing forth a transition, etc. I like the direction being taken so far as it definitely feels like a new Windows OS. The most important fact here is that it is more than the start screen.
P.S. I kinda don't like the full screen metro apps thing. One of those looks cool things. I'm just too biased towards resizeable windows.