I've always found the 12-24 to be excellent in every regard, and especially so considering what it manages to accomplish in combination with its rock-bottom price.
Mine has always been very sharp. It's not "prime sharp", but it's not a prime. The 14 L is more vivid and saturated, but that's not because the 12-24 isn't. Amazingly, the 12-24 leaves the 14 L (and just about every other rectilinear lens that even dares to come close to that neighborhood) in the dust when in managing distortion. There's so little as to be ridiculous, let alone for 12mm. And, in terms of comparing how wide a view each can grab, 14's wide, but 12's something else altogether. And sure, there are those who'd dismiss that significance by stitching together a panorama, but for that trouble, why not REALLY go stitching and use a 24 L that has very few known vices.
Maybe I've just been fortunate, but each of the 2 14 Ls I've owned have been very sharp. And while the corners always tended to improve down to f/8 or so, I've never considered either of them to be inherently "poor" as the reputation the 14 L seems to carry. In reality, I suspect it has always been judged more against its price rather than inherent performance, but YMMV.
Is the 14 L II improved? Yes. Appreciably? I suppose when wide open. Is it worth the price difference? Sure, if you've got the cash and have a proclivity for showing off sharp corners wide open. Otherwise, I've found the character and quality of the images between the two 14 L versions to be very similar.
As for the 12-24, there have been reports of QC variances, but the one I own and the two I used before buying mine were all similarly excellent. Focusing at such wide angles when shooting to emphasize perspective is finicky by nature, so maybe that accounts for a lot of what gets passed on to the 12-24 in the form of a bad rep. I dunno. I've always been very pleased with mine, as I've always been with the 14 Ls I've owned. Again, YMMV.