The standard graphics package is great and quick to make simple figures like plot(x, y), hist(x), etc. However, as soon as you want to split your data according to one or more factors (e.g. different colors for points corresponding to different treatments), then ggplot2 really shines.
Though learning the grammar of ggplot2 takes a bit of time, it's logical and in the long run is much easier to use than lattice, in my opinion.
Thanks Hadley for such a great package.