I am currently studying acouple of OU courses in natural sciences - I have not yet decided whether to pursue a degree or not. I have heard from various sources, including my tutor, that employers regard OU degrees very highly - not least because many students manage to study whilst having other commitments (job, family etc). Many OU tutors are themselves lecturers at brick universities. With regards to science degrees, I think its true to say that OU students cannot compete with students at brick unis in terms of laboratory experience. - Science students at brick unis are likely to spend many hours in the lab every week whereas OU science students are lucky to complete a week's lab work at residential school every year. Yet, the quality of science material (textbooks, DVDs, multimedia activities) is very high indeed. I also have the opportunity to attend tutorials to meet my tutor and fellow students. All in all, all OU course materials are very conducive to self-study so, in theory, its possible to study entirely by yourself. - However, there are many opportunities to involve yourself in group study - having regular contact with your tutor, attending tutorials, participating in online conferences.
Hope this helps!