Light weight, sharper than the f/2.8, one of the least expensive and best 'L' lenses.
You may want the bokeh that the f/2.8 can create, for artistic portraits. The IS on the $1600 f/2.8 is very convenient. Also, mine came with a tiny spec of dust on one of the internal elements (aargh!). Oh, and odd filter size (many pro lenses use a 77 or