This question was posted a few weeks ago, and I could not resist addressing it in my blog, since I got engaged and married during my PhD program as well. If I were to summarize my answer in one word it would be this: support.
I know many students who got married (and a few who got divorced) in graduate school, and it all comes down to how well you and your spouse can support each other during this intense time. By support, I do not just mean financial assistance, although in graduate school many students rely on their spouses to pay for the basics, especially if they don't have a stipend. An equally important type of support is emotional guidance. When you go through such an uncertain period in your life (e.g. When will you graduate? Where will you work?) the attitude of your spouse can make or break your spirit. Some students I interviewed for my book said that their spouse was their best friend, and talking with them about their issues in graduate school helped them overcome challenges and stay motivated. On the other hand, a spouse who gives you a hard time about working long hours or lack of money is probably not the best partner during graduate school. It is also very important to be clear about your (and your spouse's) work hours, and how you will make time for each other. If you are considering marriage, you probably have a good idea about your partner's attitude during times of stress.
Assuming you give yourself the green light on marriage, it is time to think about is the wedding itself. Do you have time and money to plan such an important event? Weddings (in graduate school and life in general) can range from elaborate events with hundreds of guests, to modest dinners after tying the knot at city hall. Coordinating the details of the wedding and financing while keeping your own family and future in-laws happy could be more complicated your thesis. It is up to you to decide whether you have the time and money to have the wedding that you want while you are in graduate school. I had a medium-size wedding, and I know from personal experience that planning a wedding is time-consuming and expensive. I also know that my disposable income was not significantly higher straight out of graduate school, because I was a postdoc and it was time to pay back loans and pay real rent (instead of student housing). Some students also purchase houses and cars and their disposable income is even less than in graduate school. With the troubles in the economy, many companies are down-sizing and the workload per person is increasing, so you cannot always count on having more "free" time after graduation. On the other hand, while you may not have much more time or money, post-graduate life is usually simpler with the burden of your thesis off your shoulder. :)
Of course, you may have your own reasons to get married in graduate school or wait until you finish, but I can tell you that transitioning from the wedding mood back to your thesis can take a few weeks. It is common to feel drained after your wedding, so be patient with yourself and your spouse as you start your new life together.
Wishing you the best,
Dora Farkas, PhD, Founder, PhDNet