I've always admired Sommer's work. As David puts it above, he stretches the medium with his drawings on the negatives. And those forms are so figurative, and yet abstract. I just LOVE looking at them.
The chicken heads, and the severed foot are, I think, disturbing for the subject matter, but they are so extraordinarily beautifully rendered that he quite masterfully elevates them beyond the "yuck" factor. They are all about life, and death, and though the idea of life and death are quite commonly found in a lot of art, I find his particular vision of it unique among 20thc art.
FWIW, I think he's the type of artist that may take awhile to appreciate. When I first encountered his work, it didn't do much for me. But then, I started looking at it again and again, and it just keeps intriguing me. Just give him some time, it's well worth the effort to get to know his work.