Most reviews of the 16mm f2.8 pancake lens are not very favourable. Photozone gives it one and a half star out of five, for optical quality, which I think is a bit unjust. We need to keep in mind that this is not an expensive lens; it's built to be small and therefore some compromises are made for optical quality, not to mention it's a ultra wide lens. From the samples I have taken, the edge is acceptable at f8. If you must have razor sharp corners below f8, you should be looking at the likes of Distagon 21mm f2.8
I happen to like this little lens. It has very good center sharpness. The small size, when paired with the NEX-5, it feels nice and balances well. The only "complain" I have, is that this lens is too wide for everyday shooting. I think a 35mm equivalent focal length would be ideal. Imagine a 35mm f2 equivalent lens on the NEX. I won't even need the Fuji X-100 any more!
I am sure I learn more about both the 16mm f2.8 and the NEX-5 after some use. So far, I am relatively happy with the NEX-5 and the dual lens kit.
The Ontario College of Art & Design building -- NEX-5 & 16mm f2.8 Pancake lens at f8. Click to see larger.
Old Church -- NEX-5 & 16mm f2.8 Pancake.