It's getting very popular in this generation of websites, as it's designed to work on Mobiles as-well as desktops and tablets. But do you use it for your website? I use it for one of mine.
No! ...because like every other fat bloated steaming pile of grade A farm fresh manure known as a web development framework, it bloats out the page for NOTHING of value, and in general defeats the ENTIRE point of using CSS in the first place. Developers are dumber for it even EXISTING -- and much like HTML 5, jQuery, and a whole host of other crap that's become trendy the past few years, I cannot fathom how anyone could be ignorant or just outright stupid enough to use any of it by choice! Much less it completely fails to make anything easier -- it's just more to learn to make more code than if you just learned to do it properly in the first blasted place. The only way anyone could see a legitimate benefit from any of it is if they give a flying purple fish about accessibility, or still have their heads wedged up 1997's ass! See all the re-re's who until a few years ago were sleazing out HTML 3.2 with 4 tranny on it, and now just wrap 5's lip-service around the same outdated, outmoded, and just plain broken methodologies. EXACTLY the target audience for this type of nitwit guano. How ANYONE could conceivably think it makes development easier or faster just blows my mind... the only possible explanation being said dev's not knowing enough about HTML or CSS to even be building websites in the first place. ... most likely the type of ninnies who start dicking around in photoshop before they even have semantic markup and logical document structure of their content! It is the antithesis of sane, rational, clean, accessible and minimalist development. I meet whoever came up with it in a dark alley, bad things will happen. Remember, just because something is popular doesn't make it good -- See Justin Bieber, Sarah Brightman, and vampires that sparkle!
The only thing I disagree with you is jQuery, for not all JavaScript is cross-browser supported (though I think is important that you know JavaScript first before tackling jQuery) and by using jQuery you at least know it will look good across the spectrum of browsers that are out there. Don't get me wrong I know where you are coming from, but sometimes companies insist that you know a particular framework and if you know HTML, CSS and JavaScript then learning a framework isn't that difficult. You have two choices either live on your principles or live by earning a paycheck - the choice is yours.
Ah, the cross-browser BS, of which is what, 5% of jQuery's codebase?!? 95% of what jQuery does is "gee ain't it neat" crap that doesn't belong on websites in the first place or is CSS' job! JS should enhance a page's functionality, not supplant it -- but try telling that to the re-re's who sleaze out scripted tabs, those stupid malfing banner rotators, scripted menus, and all the other inaccessible broken crap that does NOTHING to actually make the page useful to visitors and just makes it take ten times longer to load. Funny statement given that in my experience it usually just pisses all over the pages accessibility, functionality, usability, and in general I've not seen a website that hasn't been turned into total *** by it's very presence! It is at best a crutch, and at worst a sleazeball way to encourage -- again -- "Gee ain't it neat" bullshit that has NO BUSINESS on websites in the first damned place. Which is where most companies are run by dumbass ingorant ******** ****s who NEED to start being called dumbass ignorant ******** ****s so we can thin out that particular pool a bit. Though I gave up years ago on working for people who try to micromanage what they hire you for. If they think they know enough to tell you what tools to do the job with, why the **** aren't they doing it?!? As I yelled at my last full time employer, "You know jack **** about this but have the cojónes to tell me how to do the job? If you think you can do better, YOU DO IT! Otherwise, what the **** did you hire me for?!?" Some headaches are NOT worth a paycheck. See most web development where unless you are running your own company, you'd make more money saying "Ding, fries ready" -- with a fraction the stress.