I have used several frameworks and what nots, and typically like harry often find myself in a situation where its quicker to start my css from scratch however, for quick development work frameworks are very useful in the time saved, its just having the knowledge to remove what you don't use before the project goes live.
As for Em's .... for me personally the main reasoning before for em based typography/layout was for the ability to scale up your font sizes proportionally to that of the text size selected in the browser and or for fluid width web-projects, however, as the majority of modern day browsers now zoom the page rather than the text size by default, greater "general" control can be achieved now using px (pixel size measurements) inmho.
px over em has always been a confusing thing for new web developers/designers.
that said, I can appreciate great typography, and do encourage people to give both methods a go to see which one feels right for them.