I am an amateur photographer so my review is based on my needs as an enthusiast. I bought the lens used here on FM for my D600 and this lens has been the best value I invested so far in photography.
Build quality is great without being ridiculously heavy.
The lens provides the widest available focal length for FX and it's much wider than it's competitors like the Tokina 16-28, Nikon 16-35 and even the Nikon 14-24, that's a huge plus in my opinion, especially that it controls distortion better than the others. Please note that I only owned the 16-35 and the Sigma so my comparison with the 14-24 and 16-28 is quite subjective based on online photos.
The lens is sharp, really sharp. Maybe not as sharp as the 14-24 but for my needs I wouldn't justify the $1500+ price difference. It's sweet spot is F/9 and it's sharp all across the frame. Maybe Sigma has issues with QA and that explains some complaints about sharpness (this has been the case BTW with the Sigma 10-20). So I suggest if you are buying used, since it's discontinued, to ask for sample photos taken at different apertures before buying.
The no filter issue is actually not an issue for me since I knew that before buying it. It's like saying the only negative of the nikon 105 2.5 AIS is that it's manual focus which is unfair. Besides, there has been many solutions for using filters for this lens, you can google this. The same applies to comments about it's speed, you knew that before buying it and it's not a big deal since most UWA shots are taken at small apertures except in rare cases.
The only downside for me is flare, it's doesn't handle flare very well. My copy produces blue flare particularly when street lights are in the composition.
All in all this lens has been a pleasure to use and I definitely recommend it to anyone using a full frame camera. On the other hand, if you are using cropped sensor then I wouldn't recommend this lens since there are better and wider options like the Tokina 11-16, Sigma 10-20, Nikon 10-24 and many others.