Its been a few weeks now - and I've picked up the "feel" of this lens. Here are a few comments/observations.
Note! This is all based on my personal experience with my combo t2i + Samyang.
Firstly, this lens is a specialty item for the patient skilled photographer. It is NOT a tool for "grab shots". I would not recommend this lens to a soccer mom, or a weekend dad style shooter.
The Samyang is a manual focus lens. On my t2i the only way to get a sharp shot is using Live View, and manually focusing. You need to understand from the start that "live view" is misleading - and what you see is not what you get as far as exposure! The image on the live view screen may appear properly exposed - but when you take the shot it will be dark - or blown out. You need to use the light meter at the bottom of the screen to see the actual exposure.
My experience with the lens seems to indicate that it is about 2/3 of a stop DARKER then a properly exposed canon lens. I shoot in M mode, set the ISO speed (not Auto), and then manually fine tune the exposure to be 2/3 of a stop (+2) over exposed according to the light meter. You need to do this having the green box over the area you want to properly expose (the face usually in a portrait).
Another option is shooting in AV mode, setting the EV +2. You need to make sure the camera is exposing for the face of the portrait (or the part of the picture you want) by placing the green box over that part. You CANNOT use TV or P - as the camera has no access to the aperture settings on the lens, which are controlled manually. Needless to say that you cannot use any of the auto modes.
For night photos, or other kinds of shooting, I often "underexpose" and with the Samyang this would mean properly exposing - which is like -2.
It is easier to lean the camera on a table or chair top, or use a tripod, as you need to be steady to manually focus. I actually got pretty good at shooting free hand, focusing quite quickly (you get used to it), and shooting sharp shots even wide open.
You need to also note, that shooting wide open at f/1.4, assuming you are taking a portrait, will require your model sit very still. When I shoot a portrait, I set the exposure and start focusing while the model is still relaxed and moving a little, and make them sit still just for the last few seconds in which I fine tune the focus and shoot.
For shooting portraits I found the best setting as far as lighting, is soft daylight, falling on the models face. I shoot wide open f/1.4 - and if the focus was good - it gives awesome shots. The lens tends to punctuate "colors" - and you need to be careful to use a "neutral" picture style - having the saturation at 0. I also found it best to set the WB manually under the actual light using a grey card (I use the inside lining of my camera bag which is light grey). Otherwise, I seem to get a tint of "amber" in the pictures.
The Samyang is an awesome night photography lens. The brightness and colors are great.
On my t2i crop the Samyang has the magnification of a 135mm FF lens. This means you will need a longer working distance from your model, especially for a head and shoulders shot. This means, that the DOF is the FF equivalent of f/2.24. This is still quite shallow, so you get special BOKEH.
The compression affect of the lens, which is dictated by the ACTUAL focal length, is the same as the FF 85mm equivalent. This is especially important for wide faced people, who you want to stand further away from when shooting a portrait.
Shooting live view eats up lots of battery power, so you need to make sure you are fully juiced. Best to have extra batteries at hand as well.
Shooting portraits in natural daylight requires skill, to position the model in a way the light falls nicely across the face. You want to use good bright - but "soft" light - not direct sunlight or glaring light. I just bought 2 reflectors which I use to add some "fill light" to the models "other" side - the one not facing the window. The reflectors can also be used to add brightness to the face.
For portraits, try and use a spot close to a window, and have the model sit in a way that the window is facing the side of their face you want to punctuate. (not facing the window directly). Hold a reflector opposite the window to brighten the other side of the models face. Try and use a spot where there is a few feet between the model - and their background so you can blur the background.
I found for a portrait, that the lens is super sharp even wide open at f/1.4. Stopping down to f/2 might provide a slight improvement. If you want to make sure the models ears and back of head are in focus, you might need to stop down to 2.8 or 3.5. If you stop down - watch the exposure - you might need to go more then +2.