I used OpenOffice for a few years without too many problems - little annoyances here and there. Then at some point my PC would slow down to a crawl. I then found out that after exiting OO, it would leave a process running, and that process would use up nearly 100% CPU. Open a few documents a close several times and the processes would add up. I talked to a few OO users I know and they had the same problem, so it wasn't something odd on my PC. That was the last straw. I don't have a tendency to suggest a bug in open source software - had too many "STFU and RTFM or fix it yourself!" (I'm not joking.) You try to help improve a product and you get shot down, soo... Hey, call me when you grow up.
In the meantime, I got a chance to try Office 2007. Unlike many people, I got used to the ribbon pretty quickly, and it also works very well and has an improved grammar checker (after years of stagnation.) So I uninstalled OO have been running Office 2007 for nearly two years now. And now we've upgraded to it at work as well, so I'm dealing with the same software at home and at work.
I still use a lot of free and open source alternatives in other domains, but I was tired of dealing with the little quirks in OpenOffice. I still think it's a very decent suite - I do recommend it to people who don't want to pay for/cannot afford Office, though I mention that little CPU usage problem I had so they can keep an eye on it.
Anyway, I don't know how "common" a reason this is, but I do know a couple of people who have moved to Office (or the WordPerfect suite in one case) for the same reason.