Thanks to FX Guide we have some great X-MEN: FIRST CLASS images focusing on the visual effects work for the film by fx houses Rhythm & Hues, Cinesite, Luma Pictures, Digital Domain, MPC and Weta Digital. Below are some select quotes from the article.

We used reference from Enter the Dragon with Bruce Lee, in the mirror maze. That showed the imperfections and the hues and mutual density that we needed to add in. We reflected CG doubles into the mirrors themselves. They're fighting each other and there's breaking glass which was done with Maya and Houdini, and then composited in Nuke.

"We thought we might be just replacing [Beast's] mouth and a bit of the nose, but we quickly realized that to accommodate the dialogue we had to replace the whole thing. The eyebrows needed to move and the eyes were changing, and as he was wearing glasses, we had to match the nose movement."

When [Sebastian Shaw] absorbs nuclear energy from the submarine, you could see his face distorting and meta-morphisizing into this monster. You see him transform up close full screen and then turn back into a human photorealistic Kevin Bacon.

We went through a lot of development of trying to get a digital fire and smoke sim because he was more closely aligned with the devil. It wasn't a full-bodied flame but had to be very quick.

One of the problems we had was having all the facets on the surface that had a certain thickness and size, so it was hard to read the facial features, even if you added CG eyes. We distilled down what really made up her features – her eyes and lips and eyebrows and hair, and then went about building the model from scratch in 3D.

[Darwin is] trying to contain this energy from within so he's reactively swapping between all these different material properties. He goes from human to metal, which starts to heat up. And then he transitions into stone thinking that will contain it.