I have a chromebook at home and it has replaced my "serious" laptop almost entirely. Everything I do at home other than games can be done on the chromebook and it's so light and fast to boot I don't bother even opening a browser on my big laptop. Everything done in the browser gets done on my chromebook and only when I want to play games / use windows-only software do I boot up my other laptop.
I have the first gen samsung chromebook, that was priced at I think around ~350$. It's getting old and slow, but I still prefer it to my i7 laptop. It's all flimsy plastic and the power jack is starting to give up, but I'm going to use it till it gives its last breath.
I can't do anything more than 'consume content' with a tablet; not without adding an external keyboard, at which point you're basically creating your own chromebook type thing anyway. Recently I got the Dell XPS 13 with Ubuntu, and that actually reduces my tablet usage a lot: when reading with a tablet, on, say, a site such as this one, I invariably end up wanting to reply but loathing/dreading the thought of typing stuff in on the screen which is S....L....O...........W. With the new computer, which is more comfortable and portable than what I had before, I can just use a real computer rather than the tablet.
What sort of casual work would be better on a tablet than on the chromebook? Answering emails would be better with a dedicated keyboard, as would navigating in a web browser.
For casual use otherwise (movies, books, angrybirds/candy crush style games), then a tablet is probably a better tool.
What about casual programming? Still kind of a sore spot for tablets - if I'm wrong, please tell me why! If I could just do simple emacs/bash stuff with perl/python/javascript/html and maybe even C/C++ I'd be a lot happier buying a tablet. Not really sure if you can do this with a chromebook either, though. Can it run a modern linux distro?
I run Crouton with Ubuntu Raring. Javascript/HTML/CSS dev you can even do under Chrome with some web IDE; the rest works fine under Crouton. There are just a few 'exotic' things you won't get working; for instance GHCi is broken on Raring ARM.
It's probably not quite what you're looking for, but I've been experimenting recently with using the Prompt app from Panic on my iPad Mini to SSH to a Debian virtual machine on Joyent (I have some spare credit with them I wanted to put to use up). For me, using VIM this way works pretty well, especially when paired with a bluetooth keyboard.
casual work=>tablet,
? => chromebook