Their main sales pitch is "given that it's a bunch of x86 put together, you don't have to port your code to get Massive Paralellization by Intel (TM)". Some supercomputers use them, but it's true: I've never seen a worthwhile comparisson between that and a strong (or equivalent) GPGPU.

There are some very interesting talks by Intels experts on how to use these guys with AVX and AVX512 (or however its called now) and the multiplicative improvement both multicore and AVX give when used together (the sales line goes along like this "10x for multicore, and on top of that 4x for AVX, voilá 40x speedup!").

I don't work in an HPC environment, so I don't really know if they stand up in reality to Intels claims.

Hard to get? For better or worse TTBOMK there are more Intel Xeon Phi machines on the TOP500 than GPU accelerated systems.