Lovell beat me to it.
As with most 2.8 and faster lenses, the last two stops aren't really useable if you want to maintain absolute sharpness. Though, as with most lenses, the last two stop aren't useable either, so it still nets you a little better results. It's a good lens, though, and fine for trips and other daytime/outdoors scenery.
Myself, I'd buy the 1.8 instead. It's sharper because you can actually get good results if you use it at 2.8 or 4.0, while the 2.8 just isn't razor sharp at the edges until about 8.0.