I used it for a little while, about a month, while I was using Windows 7. I've been using OpenOffice for about 6 or 7 years now with only using Office at school (currently version 2003).
I found it pretty nice. I liked a few new features, such as the citation manager, although I can get extensions to do that in O

. Outlook 2007 froze on me more than anything else when I tried to use it with my GMail IMAP account. Featurewise it was nice. However, some of the only features I liked in Office are missing now, such as Autosummarize in Word.
Also, the ODF import/export filters aren't that great. I had problems moving documents from my netbook with O

to my laptop with Office 2010 that only had minimal formatting. I had to save to Office format from O

and it worked fine. But then again, I guess this isn't an issue for everyone.
Well anyway. I didn't play with it tooo much. Mostly just used Word and Outlook for about a month along with one or two things in Publisher, Powerpoint, and Visio. My family is really liking it thought as I put it on all there machines.
But I've come running back to OpenOffice.org. I'm too used to it I guess!