Currently i can say that irix 15mm is my absolute favorite. I had had the lens 1 week after release (amazon.de was slow to deliver) Compared to samyang, beats it really easily. Not only it on my opinnion really supports the 5dsr nicely, it has couple other good things.

I do mostly nightime photoshooting, and so far, haven't had any issues with weather conditions. Samyang and even my canon 24mm II gets really easily wet when there is the nasty humidity conditions, never ever had any with irix. Works also really well on low temperatures, no frosting on the lens yet. The build quality is really good, and what i do love is the infinity click and lock the lens have. So yeah, what i have used the irix 15mm blackstone so far, no need to have anything else (well maybe the new sigma 14mm f/1.8 art to get more variety to nightime shooting)