This is an excellent program (for some people). I applied to it for next year with the prospect of postponing starting my CS Ph.D. for a year to learn math/cs at oxford. Many PhD programs seem to be open to letting students defer for a year. The course selection looks great, with many relevant math, applied math, and cs theory courses. I also really like that students do a four month dissertation/project after taking two quarters of classes. Finally, I like that it seems like the best program for someone in CS who likes math, or someone in math who likes CS (a lot of advanced computing UK masters programs look very industry/applied cs oriented).
From a CS perspective, I think it is a good preparation for Algorithms/Theory/Complexity research. Some professors I talked to also said they agreed with this and they knew some grad students who did MFoCS for a year and it turned out well for them. It might also be a good program to strengthen a candidates shot at top theory program's. However, any Ph.D. program will likely have similar course offerings, which would be guaranteed to count toward the requirements, etc, so you have to really want to be at oxford as well, haha.
From a Math perspective, I don't know very much. The UK has several excellent one-year math masters programs (cambridge part iii, imperial, queen mary, kings). It would be great if someone could post a math opinion as well. I think the program seems good, but it might not be foundational enough. In the sense that you might be learning too much breadth and not enough depth in core math subjects.
One downside is that for international students, it will be quite expensive. So keep an eye out for the prestigious fellowships like marshall and rhodes because they usually have deadlines more an a year in advance (at my undergrad, it was due in july 2010 for a fall 2011 entry to oxford!)
I'll let you know if i get in and if I choose to go!