2) I have a Nikon D5200 and it serves me well as a general walk around lens. I usually carry this with a fixed 35mm one, very lightweight and versatile.. I also got it from the previous deal at buydig for 319$.
In Abode Lightroom has a lens correction profile that can fix some of the vignetting/CA problems also
This lens focuses very fast and accurate on my D5200, while my Tamron 17-50 f2.8 can get it right about 80% of the time. Also Sigma is Made in Japan, while Tamron is Made in China.
^ Tamron is made in Japan