I got a Chromebook for Christmas after 'hinting' about one for ages. So far I've been really pleased with it. It's super quick for booting up and powering down, I think official figures are quoted at 7 seconds.
Once it's up and running I've found it to be more than adequate for pretty much anything you throw at it. I've had 10+ tabs open and it doesn't seem to slow down. It plays Youtube 1080p video without issue too.
Pop an SD card in and it opens up a window to view the contents and images are opened up quickly when you double click them.
Hardware wise, the gestures take a bit of getting used to but I honestly prefer the method used on my Chromebook now to a traditional right click! I have the Samsung Series 3 and it feels well put together, although it is entirely plastic so don't expect too much, this does make it incredibly light though! The screen is okay, the viewing angles can be a bit iffy but I don't you can expect a Retina display. Couple of omissions from the keyboard that frustrate me sometimes though: Page Up/Down and a delete key (it does have a backspace though).
It's not a replacement for a PC, at least not for me. I still have a lot of stuff I use Windows for, after all the Chromebook is essentially just a web browser, don't expect to be doing anything that requires a great deal of power. It's certainly good enough to replace my ageing HP laptop as I really only use that for browsing/music etc.
Apps wise (if you can call them that) so far I've really only stuck to Google applications and as you'd expect they all perform great, just as they do in Windows through Chrome. For me the best thing is having all my stuff on both my Chromebook and my Nexus 4, which I suppose is all down to how good Google's services are.
I'm a huge fan of both the Chromebook and Google though so I am a bit bias. Hope this helps.
Oh and you can also get Chrubuntu on the Chromebook now giving you a fully functional laptop if you like. It can be dual boot either from the SSD or a SD card (which is the route I'll be taking).