I've been vaugely interested for years, but I never quite had a good handle on what problems D3 solved; it's lower level than a charting library - to the extent that you use it to manipulate SVG primitives directly. At that point I figured you may as well go all the way and write it all yourself.

However now I've started playing around with it, I realise its killer core feature: it manages the relationships between the scale of your data and the coordinates on your screen, which saves a ton of awkward calculations. Add in helpers built on top for things like axes, line smoothing etc. and you've got a hell of a powerful API for creating charts, custom visualisations and anything else you can think of. I'm becoming a fan.