Corners are so vignetted and soft I sent it back!

This lens looks great on paper--it's fast, it doesn't weigh a ton, it is mechanically well made. But when I ran some tests on my Nikon D40 I found a couple issues so serious I sent the lens back. I don't think it was limited to just the one I bought.

1) the corners were noticeably darker than the rest of the picture (called vignetting)
2) the corners were very noticeably "soft" or lacking in sharpness
3) has terrible pincushion distortion

This was especially noticeable at the wide end but that happens to be part of the lens all of us use a lot (and I use almost all the time).

I took some photos of paintings at MOMA with it and it was great for being an f/2.8 lens, so there was plenty of light to hand hold. But the straight lines of the frames and edges were always curved! And the vignetting was annoying if not completely fatal in every shot.

I might put up with these flaws in a kit lens--it's not like it's completely dysfunctional. But in an expensive premium lens, even from a third party company like Sigma (which makes some wonderful lenses, I know and have used them), it just isn't worth the money. I didn't compare to the other off brands, but I have used the Nikon version of the same lens (which is expensive) and it is startling and beautiful to behold. I'd say it's worth the extra money if you can afford it (lenses keep their resale value well, unlike most cameras). Otherwise wait for some improvement.