My experience is different; the 16 is an excellent lens in my view. True, the corners may not seem as sharp as the center at f2.8-f4, but that says more about the center than about the corners. The center is wicked sharp. At smaller apertures, corner sharpness catches up, exactly as you would need it in practical applications. Emphasis on the center when shooting at large aperture (e.g. portraits), emphasis even over the frame when shooting at small aperture (e.g. landscapes)..
The corners aren't as sharp, and there's extreme softness particularly at f2.8. As you stop down the aperture, that extreme softness is pushed further into the corners until you hardly notice it, and by f7.1, it's gone, at least on my lens. Not that the corners are ever super-sharp, but it's the blurry-softness that might distract. But most of the time, if I'm at f2.8, I have other problems (low light!), and soft corners just don't rank that high as a problem. Not to mention that unless it's really blurry, and you have a subject there, soft corners don't necessarily impact the photo, IMHO. Not to say that you couldn't possibly have a photo ruined by soft corners, but when it's typically grass, pavement, sky, etc., it just is hard to notice until you go hunting for it.
I have one photo that I printed at 8x10 that used f6.3 -- so how did the blurry corners look? Well, they were nonexistent after I was done cropping for the 8x10 ratio. The thing is, saying that there are "soft corners" doesn't describe how deep into the photo the effect goes.
But there are going to be those that won't find that acceptable. This is something each person is just going to have to work out for themselves. Meanwhile, the 16mm is the cheapest Sony-made Nex lens. I think it's a fun lens, and with the center-sharpness so high, I don't have a big problem with it. I know about the corners. I've had worse.