[tristanj]

Not me personally, but Imgix uses Mac Pros in their datacenter for graphics related work.

http://photos.imgix.com/racking-mac-pros

And not directly what you asked, but a thread that comes to mind is this one, where an unnamed company (presumably Mathworks) built a rack of Macbook Pros for testing purposes.

https://news.ycombinator.com/item?id=9967357