Open Office is pretty good for most every day purposes. Like other people, I found a few compatibility issues. If I wrote something in Open Office, saved it in Word format and opened it in MS Word, it wouldn't look quite the same.
I have to say, I think MS Office is that bit better, especially MS Office 2007. When I went back to uni, I got a discount version, which I thought was well worthwhile. There are some applications that require MS Excel and won't work with the Open Office equivalent. You've also got VBA (visual basic) with the MS versions. Also, there are a lot of features in MS Word, Access, Excel, PowerPoint (everything else), and since it's the industry standard, there are more books you can get to help take full advantage of it. For example, I'm thinking of using Excel to record some temperatures using a USB thermometer. I'm not quite sure how to do it, but I am confident I could find out. I wouldn't be nearly so sure with the Open Office spreadsheet.
Still, if Punky's got an old computer that's slowing down, it's probably not worthwhile putting MS Office on it. Open Office would do fine.