OpenOffice doesn't play nicely with SharePoint, Exchange, or Group Policy. Since these are integral parts of your environment, why introduce something that will make your life more difficult?
A few OpenOffice pros:
- Free. I assume this is why you're looking at it in the first place.
- Opens/saves as pretty much any file format out there.
- Cross-platform (doesn't matter for your intranet, but possibly interaction outside?)
A few OpenOffice cons:
- File format compatibility is not 100%
- Painfully slow
- Clunky, unintuitive UI
- No VBScript
- Excel gurus will blow up your car for making them use Calc (ditto for Access/Base)
- Default save format is .od*. Most of your users won't change this, so bring on the confusion!
There's also a lot of little things in MS Office that has made it the definitive Office Suite, like Word's fantastic templating/styling system and the slick Document Map and Outline views.
Assuming OpenOffice's feature set meets your needs, it comes down to price. Extra administration, transitionary training, dealing with users upset over losing features (through ignorance or technical disparity), extra hassle in exchanging documents, extra support cost incurred by using a non industry standard, etc. If your user base is flexible and your support system is ready to handle it (think Higher Ed), OpenOffice is probably a viable alternative.
In the situation you describe, I'd stick with MS Office. While the cost of switching will depend on how your organization values time, you're going to continue paying in small ways as long as you use OpenOffice.
On a purely personal note, I've used OpenOffice and MS Office personally and professionally. I can't stand OpenOffice for all the reasons I listed above. Microsoft's merit notwithstanding, MS Office is the industry standard because it's that damn good.