I've literally just finished an MBA (handed in my last piece of coursework today!

) at Bath. I took the modular route and have found it very enjoyable but very hard work, trying to fit in the modules and coursework with a full time job and family.
Who?'s comment about the group and interaction is very poignant. I think that as much of the value I have gained from the MBA has come from the fellow students as it has from the taught content. Meeting people from different backgrounds, with different perspectives on aspects of management is something you cannot get in many environments. There is always someone who has more experience in an area of management that you can learn from but, at the same time, everyone can contribute something.
I looked at a few other part time courses but did not really consider the OU option. Personally, I doubt that you would get the same level of networking and benefit of group experience that you would through a part time course at a reputable business school.
Wherever you decide to do it, I'm sure you will get as much out of it as possible. If you're aiming to get in to one of the big consultancies, OU probably won't do it for you whereas Bath, Cranfield, Warwick, plus a few others, will. If it's a personal journey, go with what you feel most comfortable with.
Good luck with it, but expect to work hard.