I've done two - one in Psychology and another "Open" BA.
I did my Psych degree as part of a career change whilst working. As I had a traditional degree I did get some transferred credit, which meant I could skip the non-psych level 1 courses and complete the degree in four years. In the end, I took a year out and completed the work in 5 years. Certainly cheaper when I started in 2000 and I benefitted from reduced fees for my second (third) degree.
I liked the study model of the OU. There's a lot of support if you want it - online and study groups. I didn't want that and worked away quite happily alone. The materials are all inclusive and the online access to journals is excellent. You can get a good degree without venturing into a physical library.
I continued studying with the OU as I enjoyed the model and learning. My BA was open, with a mix of general humanities then final year modules in Modern Art and Sociology, added to a couple of cross-credits from my previous course, reducing the time to four years again.
My advice- go for it! It may seem like a long slog, many employers value the commitment shown to gaining a degree through the OU. If you're working, you can continue to earn and progress at the same time. You'll be in a better place financially than a traditional student and have six years of work experience.
My only question is what you want to do at the end of the study. Will the degree be relevant? How easy is it to get a job with your chosen degree?