From my look at it (and although I'm excited by the concept, I have my doubts whether Google can execute), it's aimed squarely at the netbook market, and at promoting cloud adoption for Business and Education clients.
Anybody who has used a netbook knows. This would win hands down. The tiny screens and the tiny keyboards are utterly horrible to use. And Windows on a low powered netbook. Painful. Just painful. And what do most people use a netbook for? Any guesses? A full size screen and keyboard for little over $400? This is a netbook killer right here.
The Business and Education offerings were a different push that got conflated with the public launch of Chrome OS. This is Google's push to take on Microsoft. Google cannot, of course, compete, directly with the desktop. So they are pushing the cloud forward. For some businesses, and educational institutions, this will be favourable. For others it won't.
There's some people who keep insisting this platform doesn't have a market because it doesn't do everything. That's missing the point. Just like the iPad (and it pretty much has the same capabilities), this does most of what the general public wants. Need more horsepower? Get a bigger engine (computer). Some people might find this unusual...for example, all you graphic artists seem to have Mac Pros. But many of us (say engineers who might use UNIX CAD stations) find it perfectly normal to have a light box for email and web and a powerhouse (often a shared resource) for heavy design work. Chrome OS is targeting that web and email box. The obvious difference between this and a tablet being the fact that it functions like a full-fledged laptop.
And also talk about what apps will it have. Again, missing the point. Apps may be developed down the road. But initially this is aimed at companies and educational institutions. If they want to deploy a solution like this, they'll obviously be making sure the apps they run are available (or will probably build it themselves).
I don't know where you folks work, but in my office, I have an XP machine on my desk that's about 3 years old. The thing takes forever to boot up. It's horribly slow. And I'm stuck with IE6 (because that's what the IT folks give you...actually we just got IE7 two months back). There's a lot of the world that lives like this. If you're lucky enough to get a new machine every year, good for you. For the rest of us, a box like this would be a step up. It's not going to be appropriate for every workplace (being military....mine is out, for example). But there's a lot of workplaces (particularly places which rely largely on Google Apps today) which would find simple boxes like this to be a good bargain.
Finally the cost. There's a lot of focus on the subscription model ($20 for education users, $28 for business users). That's just one avenue for adopting Chrome. A lot of users will probably choose to simply buy the devices outright through retail or wholesale channels. The subscription model is more an extension of Google Apps. It's Google extending its SaaS business in to the hardware realm. They aren't selling you the machine. They are selling your a hardware provision and management service. But I doubt it'll become the primary way of selling Chrome laptops in the future.