I recently bought a Samsung Chromebook. I posted my thoughts about the hardware last week. Following are my initial impressions of Chromium OS.
Chromium OS is a first blush a strange idea. An entire operating system focused on one interface: a web browser. And if that's all it were it would be strange. But I'm beginning to see more.
Chromium OS on a Chromebook
Before I bought the Chromebook I thought I'd try it in a virtual machine. That didn't go very far and probably for good reason. I probably would have hated it.
While technically you can separate Chromium OS from the Chromebook it's a bit like running MacOS on non-Mac hardware. The two are built to go hand in hand.
The first thing you notice when starting up a Chromebook with Chromium is how fast it boots. Part of that is the SSD drive. But I think there's more to it than hardware. I think part of it has to be an OS tailored for one a specific set of applications and hardware.
Think about MacOS. You don't typically install MacOS because it comes on a Macintosh computer. The OS is built from bottom up to support a limited amount of hardware. All the drivers are written and ready to go and they all come from Apple. You get optimization that's just not there for franken-systems.
Chromium only has to support a subset of what's available.
Simple: type any Gmail or Google Apps id into the login screen with password and you're see the desktop and Launcher Bar at the bottom of the screen.
The Launcher Bar has a few applications already docked for easy access: Chrome, Gmail, Search and a few others, all docked on the left side. The final icon on the left lists other applications.
On the right side is a status and notifications area with time, battery strength, wireless status. Clicking on it reveals more options (screen brightness and others) including Settings.
Clicking Settings take you to a screen very much like the regular Chrome Settings page. From there you can set your home page, how to handle cookies and other plugins and everything else you'd expect. It is a Chrome browser after all.
But there's more. There's also a group of settings related to the Chromebook itself. You can set whether to require a password when waking the computer up, manage users who've logged in, manage wireless connections, setup a VPN, keyboard and track pad (including mapping the Search, Control and Alt keys) and a few others.
What's notable is how few settings there, relatively speaking. If you use a Mac you'd expect to see a bevy options: CDs and DVDs, Sharing, Mobile Me. One you won't find on the Chromebook is Screen Saver. The Chromebook itself decides when to lock the screen. 
The most familiar part of Chromium OS and the Chromebooks over all is the Chrome browser. Whatever you're used to doing in Chrome on a Mac, PC or Linux computer you can do on a Chromebook. With very few exceptions, all themes and extensions just work.
The Web Store
If you're like me you may have used Chrome in the past, maybe installed some plugins/extensions and maybe a them. But what about the Web Store? Did you use that? I hadn't.
In Chromium OS the Web Store is paramount. It's your number one (almost only) option for extending the functionality of a Chromebook beyond what you find on websites. Want to play games? The Web Store has them. Need an SSH client? The Web Store has one. Unless all you do is browse web site or use services like Gmail, you'll need the Web Store.
But not all Web Store life is cherries. Many Chrome Apps have native components that run in a sandboxed process in Chromium OS. Native code means compilation which means compiling for the correct CPU. There are a few extensions that won't work with ARM processors yet. Google are working on a way of delivering cross-architecture Apps but for now you'll find the odd app that won't install because it's "Not Supported on Your Architecture".
Any discussion of apps leads to what Chromium supports and doesn't.
The Chromebook does have Adobe Flash but does not have Java. If you absolutely, positively have to run Java, the Chromebook is not for you.
Yes and no. The Chromebook is Linux under the hood but you can't get to the full command line unless you run it in Developer Mode. And once you're in Developer Mode there's only so much you can do unless you add the rest of the Linux userland.
As far as I can tell, no. The Chromium OS GUI is similar to the Mac OS GUI. It's sui generis.
Yes in both the Crosh shell and a Web Store app. The Web Store app is head and shoulders above the ssh from Crosh. This is in part because you can then run it as a separate window which then allows you to use ctrl-w in Emacs without closing the window.
I haven't found a stand-alone VNC client yet. I've found a few that will proxy the connection through the servers of the company offering the tool.
I think I like Chromium. It's early days still. What I really need to decide is how comfortable I am with putting more (all?) my data on Google servers. I'll address that question later.
 Which in itself takes a bit getting used to. I'd like to be able to set the lock out or at least manually invoke screen lock.
 There may be a way to hack it on in Developer Mode but Java's not supported out of the box or from Google.