Another vote in favour; I took a masters in maths with the OU and it was a rigourous, solid mathematical experience throughout. Having done all the work and sat a number of quite hardcore exams, it's also nice to get a bit of paper at the end of it.
I'm doing a masters in Computing with them now. I'm enjoying it quite a lot, and it is definitely rigorous (though more on the management side rather than the technical side).