I do not have the 35mm version but I do own and shoot with the Samyang 14mm. My copy is the sharpest lens out of all my Canon L lenses when focusing is spot on (and when it's not, the image quality is still comparable to my 24L II...that's how sharp this lens is. Every blade of grass, every autumn leave is crisp and clean). I have shot with it both manually using viewfinder as well as Live View at x5 and x10 magnification. Because of the ultra-wide focal length, I find it superb for landscape and prefer to focus using Live View. It's fairly easy to use as I normally use a tripod with mirror lockup anyway when shooting landscape.
You may already know that while Samyang lenses are generally superb in many respect, there are a good number of frustrated users. This is because first off, it is a manual lens and as such manual lenses are different and a bit harder to use if you have only used AF lenses. It does not have focus confirmation, not on the Canon mount version and the Nikon version isn't all that great. I have installed a chip on mine. As a matter of fact, I have two different chips which I alternate using it with. I do not use the chip for focusing confirmation but rather simply for gathering correct EXIF data for organization purposes. The focusing ring on many copies are off, and I mean off quite a bit, so you will have to get use to your own copy to judge correctly distance so you can nail focus if you don't use Live View. There's a thread on POTN which I would be happy to find which explains how to do this as well as manually taking off the focusing ring and re-aligning it to read correctly.
And yes, as some have pointed out, if you're using the 35mm for shooting action shots it may be difficult to nail focus but this is generally true of all manual lenses. But don't let that stop you, I have found examples of great action images shot with this lens using VF with beautiful DOF. I am looking to own a 35mm lens and is patiently waiting for the Mark II version but the Samyang 35mm is looking like a beautiful alternative. I hope this helps.