at last Canon and Nikon users have a wide angle lens which is sharp and well corrected for most aberrations, so much so that this lens alone encouraged many Canon pros to move to Nikon whilst others bought this lens to use in manual focus mode on their Canons.
in 2014, Canon finally addressed this by bringing out their excellent Canon EF 16-35mm f/4L although this does not go as wide and is only f/4.
specs
14-24mm f/2.8 pro full frame zoom giving 114° - 84° field of view
14 elements in 11 groups
no filter (but 3rd party filter holders using 150mm filters)
close focus 28cm
9 blades
970g
98mm x 132mm long
A$3000
compared to the Olympus mZD 7-14mm f/2.8 zoom lens
the Nikon AF-S 14-24mm f/2.8G lens is generally regarded as the best ultra-wide zoom lens for full frame, so much so, that even many Canon users go to the trouble of using it (although now they have the Canon 16-35mm f/4L IS lens which is superb)
let's see how the Olympus lens compares, it is:
much lighter at 524g instead of 970g
much shorter at 106mm vs 132mm
focuses closer 20cm vs 28cm
more affordable at $A1499 compared to $A2797
sharper wide open edge-to-edge (see charts below)
marginally less CA
much less distortion at widest zoom 1.7% compared to 7.4% barrel distortion!!
if you already have a Nikon full frame dSLR with 35mp and you shoot landscapes or studio fashion on a tripod at f/8 then it may have a use (although for static scenes, the new Olympus cameras can shoot 40mp HiRes shots with less moire on a tripod, although a little less dynamic range)
one could argue you can get 2 stops better ISO on full frame, but with this lens you need to stop it down 2 stops to get close to the corner sharpness of the Olympus lens, so that argument generally won't hold much
you may get better sun stars with its 9 blades
otherwise see your mental health therapist
From ePhotozine.com's reviews (not totally comparable given different sensor sizes, and note the scales are different):