I bought the Sigma 1.4 art earlier this week from
LCE, thought it was a bargain at £349 and when it came it was like new, fully boxed with all the bits as well. Might be worth keeping an eye on their website for another. Considering the Nikon version is actually more than what I paid for the Art lens it was really a no brainer.
Advantage of the Sigma over the Nikon is that it can be updated and adjusted with the dock, it is extremely well built, super fast to acquire focus and practically silent, you have to really listen to actually hear anything.