I use OpenOffice almost exclusively. I highly recommend it, and I think you should save the 70 pounds unless you have some reason to do otherwise.
I have written documents at work using OpenOffice that I have sent to people who read it with Microsoft word. And, I have read documents that people have sent me. Again, no problem.
There are a couple of odd things that OpenOffice doesn't do well. The most important is the little notes and change logs, that you can use in Microsoft Word. This feature is used when many people are working on the same document, and you can leave notes about why you made each change. One of the few times I needed to use Word is when I was working on a project that relied on this feature.
I have also had no problems reading and writing Excel documents.
One other caviat, when you share with Microsoft users, you need to remember to save documents in Microsoft formats rather than in the (in my view superior) OpenOffice formats which Microsoft doesn't know how to read. This is very easy to do, in fact you can set up OpenOffice to always do this.
Here is my advice. OpenOffice is a very well made, intuitive, full-featured product. You should install it and use it. Then in the future if for some reason you feel like you need to drop the 70 pounds on that other software, there is no reason you can't (since you can easily move your work to Microsoft if you ever need to).