flowers, thanks for some good discussion and 20 primes, wow, I thought I had a problem ! The only thing I'm going off of in regards to Samyang is what I have read from Roger over at Lensrentals. He has said that their warranty service and repairs are so bad that you might as well throw away the lens if it breaks, at least here in the US. I see he's updated his last post about this to remove that language, but that's pretty much what he had said (http://www.lensrentals.com/blog/2013/08/lensrentals-repair-data-2012-2013). I know one thing, though, those lenses can't be beat for astrophotography - Canon's fast lenses (other the the 24-70 2.8II) all have horrible coma issues in the corners.
Oh wow, I did not know that. I've never had an issue with my Samyangs yet, so going on hope! But as someone pointed out, in some cases even two Samyangs don't make the price of one Canon/Sigma, so if both of the two lenses last for a long time... togther they might outlive the competition! But in complete seriousness, if the warranty doesn't work and the Samyangs start to need servicing often, I'll look to Canon or Sigma. Luckily I haven't had any Samyangs need any servicing yet. He did write this though: "I can give you a simple generalization, though: If you want to buy a lens that will last decades, then you want a completely mechanical prime lens." Based on that you've got little to worry about with the manual focus, manual aperture Samyangs!
Ps. Lensrentals seems to have removed the bit about Rokinon/Samyang/Bower lenses because Rokinon chose to lie to him about Rokinons being different. If you don't believe me, you (or Roger) can contact Samyang directly and ask them! The lenses are the exact same lenses as the Samyang, just rebranded.