(my) Conclusions, vs the Canon 24mm f/1.4 L II:
Canon barrel-distorts less.
Canon has better centre performance.
Samyang has better border performance (at least wide-open on FF).
The loss-of-contrast in the Samyang wide-open can put me off shooting wide-open (it's bad on the 35mm, i rarely use that one wider than f/2)
Samyang has slightly less CA.
Canon has better Build Quality (I can certainly vouch for that, based on my Samyang 35mm), and Weather Sealing.
Both vignette pretty badly, but I never mind that.
The Samyang is $700 at B+H. The Canon is $1700. Guess which one i'm picking?
(neither, until I can get a used Samyang for $400)