Poly/ML is a very fast implementation of Standard ML 97. Interesting feature (apart from performance): the system is much more oriented towards Lisp systems than towards classic command-line compilers. You have an interactive working environment and an image that is automatically saved when the session ends and thus makes all definitions available again in the next run.
The advantage of this way of working is simply that you can adapt your system to your own needs over time. Together with the FFI and external libraries, you can gradually put together a nice environment that is optimized exactly for the intended goals. In addition, you often save extra object databases: you simply save values (or in the case of ML, structures) in the image and keep them there.
Since Poly/ML simply hangs the image in the address space using mmap and does not first read and then write it later, the whole thing is also quite fast - with image systems, the system start often bothers.
Oh, and by the way, Poly/ML runs smoothly under OS X ...
The speed of the compiler unfortunately does not necessarily continue in the generated code, as can be seen from this comparison. But just like with OpenMCL, the fast environment is definitely advantageous for prototyping. You can then set a compiler that takes longer but then spits out better optimized code like MLTon.