Student Universe contracts directly with the airlines for their fares. In their contract with each airline, it stipulates that they must verify that you are either a student or faculty member. Each airline may have slightly different requirements, but AA requires that they verify your status every six months. An e-mail address with .edu is usually sufficient, but they will call the school to verify your status if there is any question.
I have been told by Student Universe that I am one of their best customers. This is due to the fact that I am finishing my Masters at NYU, but continue to work in San Diego. As a result, I commute back to San Diego every weekend. I normally pay between $280-310 base, plus taxes. Much better than I can normally get through AA.com or Orbitz. All tickets are eligible for FF miles. I can vouch for that with my Ex Plat status on AA. The fares are also eligible for upgrades. For AA tickets, there are no age restrictions (a very nice touch since at 40 I usually don't qualify under regular student fares!!).
I have found that their customer service is great and I normally recommend them to my friends who are eligible to purchase tickets through the site.
If anyone has any other questions, I'd be more than happy to answer them to the best of my knowledge.