My sister prefers Open Office and she hasn't had a complaint about it yet. She switched over because she couldn't stand Office for various reasons. The UI is a bit toned down in comparison to MS Office, but it gets the job done. I've always preferred MS Office, as it is what I have been used to for quite some time. I've used XP to the present 2010 beta. It is expensive, there is no doubt about that, but I've always found it worth the cost.
I used to use Open Office 2.x, but after getting a good deal on Office 2007 I've been using that and wouldn't go back. I find the default templates (for powerpoint) are better and the GUI is more intuitive. Yes, I do actually like the Ribbon unlike many. I never used previous versions of Office regularly, but I know they are similar to Open Office, and I don't like having 100 toolboxes around my screen to access a single tool on each when the Ribbon has most of the items I need easily accessible with a single click to change tab.