Hi there,
I'm an OU student (about to start my fourth module). I'm studying part-time, 60 credits a year, and whilst on an average week it's not too intense, around the time that assignments are due it can take up quite a few hours. Plus, if it's a module with an exam, you'll need plenty of revision time.
I'd say that it would be possible to study 120 credits a year on top of your job, but it would be tough. Perhaps you could take on one 60 credit module first and see how you find it?
Is it worth doing? Definitely. The quality of the materials is excellent. It's challenging, especially from level 2 upwards, and I have learned so much!
My modules haven't had a lot in the way of video lectures. Mostly it's just a whole lot of reading and some audio (and this year independent research), but you can interact with other students through the forums, and go to tutorials if you like, and all the tutors I've had were quite interactive and very helpful.
I'm already putting what I have learned into practice, and I haven't even graduated yet!
If you study with the OU, you'll need to motivate yourself. Although the books and the website will guide you through the module and help you build up your skills, there's nobody there to hold your hand unless you ask for help from your tutor. That won't suit every person, but if you like relatively independent learning, it's quite fulfilling. I like setting my own pace.
I've always received helpful feedback on my assignments and it's a wonderful feeling receiving a good score and, after a while, realising how much you've learned and how you've progressed. On the whole, I'm a happy OU student!
Hope this helps a bit!