I didn't even notice the serif font, because I have overridden the font-family in almost all places to the Ubuntu font-family (hooray for user style sheets!). I tried it as an experiment somewhere around a year ago. I strike the occasional website that doesn't look right and where it looks like the typeface used may have damaged it, but overall, it's marvellous. I noticed very quickly that the whole web was prettier and more readable. I tried going back, just out of curiosity, a month or two later. I went back to overriding the font-family less than a week later.
But as far as their layout is concerned, I do like the large typography, and don't see any problem with the paragraph spacing and layout. Overall, it's a nice, clean layout.
I've been doing this for a few years, along with a reasonable minimum font size. I find it really tough to go back to a smorgasbord of fonts and sizes, with some of them often unreadable. Controlling the fonts via Firefox is awesome.
The biggest downside to this is where sites (e.g. github) try using private-range unicode codepoints for graphical icons, not realizing that users often have other fonts set. I really discourage site authors for hacks and stick to using graphics for icons.
But as far as their layout is concerned, I do like the large typography, and don't see any problem with the paragraph spacing and layout. Overall, it's a nice, clean layout.