Yes, just finishing an MBA with them. I'd give them 8/10 for it. Most of my tutors have been ok,a couple have been knobs. Materials good and they do try to give you all you need because historically, you would have needed access to a decent library for supplementary material - not as relevant today thanks to
Wikipedia online journals. Any degree which benefits from practical implementation in the workplace works better part time (management and engineering are good examples) in my opinion.
As a serial degree taker, who's previously studied at 3 other universitiesI'd say the OU is pretty well organised. To get the best out of it though, you need to be too.
Lots of other places do distance learning too though, so check out what else is available and make your decision based on the quality and relevance of the course, not just the institution.