Whenever I need to use OpenOffice it feels like I've stepped back to Office XP or Office 2000 in terms of UI experience (it's not even Office 2003 yet). That said, Office XP is certainly good enough for many of your users.
What I'd be more worried about is the quality of OpenOffice's document translation, when saving to and opening from Microsoft's file formats. It's getting better, but still far from perfect. I know a lot places are able to get around this by simply identifying a few key users in every area (usually secretaries) to keep MS Office so others can ask these individuals to print or save documents where the format needs to be perfect, so it's actually not that big of a deal.
But single biggest concern is spreadsheets. A lot of places have large spreadsheets with complicated formulas that they use daily and cannot be replicated easily. You can buy Excel separately for these users if you need to and save at least a little money (the same for your Outlook/Exchange issue as well — but by the time you purchase this and maintain two sets of programs and licensing you're not saving as much anymore.)
Another option is to run parallel for a while, and request that all new documents be created in OpenOffice using ODF. Over time the legacy Excel sheets will become less important.
As for Access, if you have that many people really using it you might want to consider adding development resources to move all those little databases to real professionally-maintained applications. Again, if you have that many access databases around, hiring a real developer who's good with databases to move them all to a real sql server or mysql-backed system should pay for itself, with the first returns coming from being able to buy a simpler version of Office (std vs pro), next as other users spend less time building in Access, and finally as you are able to start mining this now-unified data to make better decisions and do things you were not able to before.
This also depends on your users. If you have the kind of user base that needs to be taught how to use basic Word features, you might want to stick with what they were trained on (though the transition to the ribbon in Office 2007 makes a good excuse to move elsewhere). But if you can expect most of them to just pick up the new program as well as a normal college student would browse a new web site, than you'll be fine.
BTW, we get around the default save location issue by mapping each desktop user's profile so that their My Documents folder points to the appropriate network share. Those with laptops are set up to use synctoy.
One last point to consider is politics. To adapt a common phrase, "No one was ever fired for sticking with Microsoft", though your concern here is the change from the status quo. If you switch to OpenOffice, you're gonna move a lot of people's cheese and not all of them will like the change, no matter how much it helps them.
The final result where I work is that we're sticking with MS Office for now (and though I'd have to sell the idea to my users, I am the primary decision maker for those issues here). However, we have a lot of those large spreadsheets, we're an educational institution that can license Pro Plus for only $70, and we run in a mixed shop that upgrades Office for a user at the time of a new PC purchase, so the cost of Office relative to the cost of the PC is actually pretty small. We do have a few users who prefer Google Docs for the collaboration features, and if we had to pay the $350 retail price or even the $200 OEM we might be quicker to move elsewhere.