OpenLearn is good in the same way that something like FutureLearn, Coursera, EdX or Udacity is good: they're all Massively Open Online Course (MOOC) platforms and therefore their courses aren't valued as real qualifications. They are however good evidence of interest and knowledge in the subjects you study, of working hard to learn in your spare time and of continuing personal development in the workplace. Some of them, such as Udacity and Coursera, offer paid-for "nanodegree" or "specialisation certificates" which claim to be valued similarly to higher education but I'm dubious about that.
OpenLearn specifically has a huge range of material, much of which is taken from discontinued (and some current) Open University courses. These range from basic introductory courses on things like arithmetic or using calculators to 100 hour extracts from masters degree level modules. I'd say give it a shot, and also try FutureLearn, which is also an Open University initiative which features courses from themselves, other universities and even organisations like the European Space Agency (their course on climate monitoring from space was a really good one).
Definitely give it a try, but don't expect to earn real qualifications.