I recently got this lens from B&H and my copy is sharp all around. There is a little bit of light fall off in the corners and on the edges at 10-12mm but this isn't as evident when shooting outdoors or when a scene is not evenly lit.
The lens really surprised me the first time that I used it. I was amazed by how much area I could actually see - it's truely a wide angle.
With the crop factor on my D70s this is a 15-30mm, which is great for story telling landscapes and wide interior shots.
There is a bit of distortion with the lens, and shots that have a horrizon line curve a bit due to barrell distortion at the wider side of the the zoom.
Popular Photography recently reviewed this lens, along with the Tokina and Tamron lenses that are similar, and concluded that this was the best of the three 3rd party lenses.
Overall, it's a great lens for the money and I look forward to using it more as the weather gets warmer here in New England.