First let me say that I had to try 2 copies of this lens before I got one that was spot on (I had the same problem with my 24-70L way back when). The first one was almost unusable.
If you manage to get a decent copy of this lens there aren't any negative points. Things like filter size and weight aren't cons, they're inconveniences, if anything. Come on, people! The problems/ cons for me personally lie in Canon's QC .... as far as I'm concerned, there's lots and lots of room for improvement here!
UWA lenses - no matter how sophisticated - will always have a certain amount of light fall-off or soft edges on a FF body, that's something you'll just have to live with, but if you get a decent copy of this lens you'll know you've got a decent copy!
F/2.8, 16mm on FF, 20.8mm-45.5mm on a 1D2, top-notch IQ and build are the reasons why I bought this lens.
I'm not gonna repeat what lots of people have already said; this is a suprior piece of optics, which costs money. If you need a UWA lens and you've got the cash then get it, and don't even consider going for 2nd best (ie 17-40L).