Back towards the dawn of OS X, there seemed to be a great deal of hubbub, at least in the Mac world (I was nowhere near scientific computing at the time) about the Mac OS as a platform for scientific computing and HPC applications.
XGrid came out of the box, Virginia Tech had their fancy Mac-based computing cluster, Stanford was doing cool things, etc. etc.
More recently however, things have been quiet. The Macresearch.org site is essentially a ghost town filled with spammer zombies, the XServe is dead, and an awful lot of the marketing literature and the like seems to even be from the pre-Intel processor area. But XGrid is still there, the whole *nix OS underpinning is there, and the platform seems to have decent support among Python, R, and some of the newer languages.
So, from people who know more about this than I do...how fares OS X? Are they viable client-side computers for scientific computing? Is using them as a server/cluster/etc. through XGrid or something like it simply a novelty application?