I've got the 16-35L II and have had a few issues.
First, my 20/2.8, 28/1.8 are sharper, crisper in the center. I've not really taken a good look at the edges.
My own 16-35L II is soft on detail at 16mm, but retains good contrast. It looks sharper than it really is.
By 24mm it sharpens up considerably, and is not too bad (but not stellar) at 35mm.
The primes kick its poverbial butt.
Falloff... not an issue on my 5Dii or 7D which are correctable for that.
My own copy shows about -6 focus correction at 35mm, and maybe (hard to tell) +2 at 16mm. My bodies will only correct for the entire spectrum of a zoom lens, so I set it at -5 as a compromise.
Its not an OMG THATS SHARP!!!! lens. Its more than adequate though, and retains good contrast and color, and low flare through all its zoom range. Flare is pretty well corrected too.
I shoot it, because its 16mm, and I find that I use that low low end of focal length quite a lot. I can still zoom in for a 35mm focal length (my favorite). It might as well be a 16 or 35 rather than a 16-35 zoom. I rarely use the intermediate focal lengths.
And yah, I'd get it again in a heartbeat if it was lost stolen damaged etc etc etc