This is an interesting subject to me as well, because I too had a rule of not dating anyone in my department when I started. However, 9 months in, things happened and I started dating someone whose office is right next door to mine. We were very good friends by that time, and it has been almost a year now, and we are happily together and moving in together this summer. No one else in the department seems upset by it, and we are a very close department who hang out a lot outside of school. But we also aren't lovey-dovey in public, and I have made clear boundaries that we don't kiss or act like we are dating when at school, as I want to still be seen as professional.
So, I think it can work. Obviously, things can go wrong though, so you need to be prepared to still work with that person. I have never had a terrible break up in the past, I am still friends with all of my exes, so I don't see it as a huge issue for me, but others have different experiences.
There is actually one old couple in my department who can't be in the same room as one another, and I think it has negatively effected both of their graduate school experiences, as well as the people around them. Their break-up was not amicable, and resulted in campus police being informed. So, it totally depends on the people.