I’ve been using the 14mm Samyang for a while now, and I must admit it’s a very good lens, very sharp. I’ve noticed on occasion that it is susceptible to lens flares, but nothing too bad considering it’s 14mm.
Using the Abode lens profile downloader I’ve managed to get a reasonable lens profile for use with Lightroom 4.4 and it’s fixed most of the vignetting and distortion.
It’s totally manual, which for landscape photography is perfect, since you normally spend the extra time to setup your shot and check focus using live view anyway.
If your considering buying one, then go for it, it’s well worth the money, or lack there off when compared to the brand equivalent.