I'm an Office 2k3 user. Which sucks b/c my current job is still running office 2k. Pain in the ass old software. I prefer office 2k3 b/c I know a metric f*ckton about it through dealing with it in other jobs. I even have a big book(~900 pages) of office 2k3 tips, tricks, workarounds & fixes. It sits in my desk at work because we do have some 2k3 users.
Office 2k7 is our next deployment at work. It will be happening once we get the licenses bought(hooray VLK), and get the users trained. I'm going to have a solid 2 months of dealing with 2k7 issues and I know it. Basically it's going to be user training because it is so different from our current setup. I still haven't been trained on 2k7, but from playing around with it, once you get into how it functions and why it was changed, it's smoother, faster and honestly, easier for end users. It's just making that shift that is so time consuming & frustrating.
As for open office, I've seen far too many compliance issues with it for it to really be viable in any medium or higher business from what I've seen. For small businesses looking to save some cash, it does work well. Just make sure that the documents are MS Office compliant just in case a conversion is done or you are dealing with outside clients who run an MS shop, as so many do.
Damn I am such an IT nerd it scares me.