While I can't speak from experience, a couple people I worked with had their girlfriends working there too.
Case1. They got along great, and eventually moved in together. Helped each other at work too.
Case2. Things started out ok, but about a month after the girlfriend started, things took a bad turn. They were both fighting a lot, and always moping about. It seemed as though they were just seeing way too much of each other, and couldn't get any space. Makes it a real pain in the ass to go to the breakroom and have to listen to them go on. They both quit, and I don't know if they are still together or not.
So, I guess it matters, as others have said, about how mature both of you are to be able to handle it.