There are a lot of downsides which never get mentioned here on the Nikon and trust me its not perfect by any stretch. I use 5 different brands of lenses to get the best images i can from it and frankly that is not right Nikon cannot get a damn good wide angle out the door when a 500 dollar Samyang 14mm distorts like a banchee but is damn sharp. Clean up the distortion and its a damn nice lens. Really whats Nikons excuse , so hearing how great it is only half the story.
Nothing is perfect...
However I think it's safe to say that Nikon makes some very fine lenses and if we talk about wide angles the 14 to 24mm is quite exceptional.
Lets see how it compares to the Samyang you praise.
At 2.8 the Samyang looks like it has a fine stocking over the lens.
At 5.6 the Nikon has better contrast and resulting in better detail.
More full res examples here:http://www.flickr.com/photos/hhackbarth/with/7638840722/#photo_7638840722
Even considering the price difference the Nikon is a good deal being a 14 to 24 zoom that replaces three focal lengths and is infinitely more useful.
Being a zoom it does have CA in the corners, but that is correctable very well.
Also if we really look at this in a realistic manner with the Nikon you have both quality and more choice.
MF has no very wide zooms. No inexpensive options like the Samyang for a focal length someone needs , but cannot justify spending more.
If some things are not covered by Nikon but done well by say Canon you can buy both and still be far below the prices of MFD. Ultra wide TS lens for example.
There are also excellent Carl Zeiss lenses and a whole new line of ultra high end lenses coming from Zeiss with the 55mm already announced and shown.
Even with these being high priced they still come in below the cost of equivalent MF lens and back.