Pros
Work on the main campus with 1000s of other people means there's lots of other activities to get involved with, outside of hours and during lunch breaks and things like that. The massive campus population means you can always find someone to socialise with too, if you don't like your immediate departmental colleagues. It being a large University there are a lot of innovative projects/research taking place, so no doubt you will find yourself doing something you can get passionate about if you're working on the academic side of things. Even outside of academia, the institution is a bit of a world leader when it comes to distance learning, competition is getting stiffer now but this should only compel the OU on further. I found there was a great emphasis on work-life balance. The University-wide budget is being cut at the moment so you may find yourself laden with a little more work than before, but you should rarely be worked to the teeth. Decent opportunities for growth and development, sometimes you just gotta ask though. Lateral movement is also often available given the OU's sheer size, and with some quite kind firing policies you should find ample job protection/security. Great relationships with senior managers in my opinion who were accommodating, willing to help, approachable and supportive. Beautiful campus and surrounding greenery.
Cons
Little boring sometimes. It's a bureaucratic beast so change can sometimes be slow but this is a key focus for improvement at the moment. The politics can also get tiresome sometimes, especially at the managerial levels, but this can easily enough be ignored.