I think it's impossible to generalise about how employers view OU qualifications: in some fields an OU degree isn't going to be considered good enough (but equally a degree from a less prestigious brick uni could be viewed the same way), whereas in other occupational areas employers will value the dedication and self-discipline that is required to achieve a degree by distance learning.
The OU's academic standards are the same as those in brick unis (again, these vary from uni to uni somewhat and from faculty to faculty), but there is no question that OU degrees comply with the same overall standards as brick unis.
The rep problem comes (I think) from the open access policy of the OU: no qualifications are required for entry. This has always been part of the OU's ethos - hence the name "Open". Taking away the competition for admission may have a negative impact on the university's reputation in the eyes of some. Open access does not mean a compromise in standards: level 1 courses (roughly equivalent to first year ug study) will start from a lower baseline than the brick uni equivalent, but the learning curve is steeper. At levels 2 and 3 the level of study is equivalent to 2nd and 3rd year ug study in brick uni, but without the daily face-to-face support of tutors and peers.