Yup, in terms of IQ, this lens just blows away the 16-35 and 17-40.
If you've still got one of those two lenses, you could try giving it away for free because they're not really worth having now.
Unless you are a professional that does events. Better corner sharpness from f/4-f/8 is not worth losing f/2.8. IS can't help motion blur.
Lets see. The trade off is 1 stop of ISO for sharp across the frame.
And if you need 24/28/35 at f/2.8, Canon now has three primes with IS.
In the range I personally have in addition to the 16-35mm f/2.8L II, the 24mm f/1.4L and a 35mm f/2 IS USM. But, neither is a zoom (helpful at receptions), and neither can do 16-23mm. I also have the 24-70mm f/2.8L II, which is a zoom - but once again cannot do wider than 24mm. Hence, the importance of the 16-35mm f/2.8L II. I don't want to be stuck with the limitations of a f/4 lens. Also, the corner sharpness is unimportant when taking pictures of people as you need to put them in the center of the frame to avoid perspective distortion anyway in most of this focal length - center sharpness is far more critical. Also, IS for my needs is virtually useless below 70mm as my shutter will usually be 1/100 to avoid motion blur.
Believe me, if I didn't think I needed f/2.8 on the 16-35, I would sell the f/2.8L II on eBay as even today it sells for more used than the 16-35 f/4 IS costs new at full MSRP - I would essentially be able to do a swap with no net loss. But, I don't want to be put in the situation where I need 16-23mm f/2.8 and have to take a compromised picture because I don't have it.
Finally, note that the 16-35 f/2.8L II is actually sharp across the frame at f/11 (and pretty close at f/8). So if you have this lens, all you have to do is stop down - which you probably would be doing for DOF in landscape anyway. f/4 is really where the 16-35 f/2.8L II gets trounced in sharpness by the 16-35 f/4 IS, but that is the aperture that would likely be least used by either event or landscape photographers - too slow for event photographers, not enough DOF for landscape.