Did a cross-country LDR for first semester of law school. We'd originally said that we'd move back in together after law school was over and I'd come back, then we said after 2L my SO would move here, then after 1L, needless to say, after 1 semester we've moved back in together and I've got an hour commute to school and back (and am paying rent on top of a dorm room I committed to for a year).
Now we were lucky enough that my SO was able to move, and I think we would have survived law school. It's hard, but I think it's doable for the right couple. We found plenty of weekends, talked for hours on the phone, and neither my grades nor our relationship suffered at all (though we personally hated being so apart, which is what made it hard, it wasn't detrimental to the relationship
). We weren't the type to get bitter at the other because the other is off working all the time, my SO works overseas about 3 weeks out of 5 and I had a professional long hours job before law school so we'd been used to a little separation so as to get mad at the situation and not take it out on each other.
I think it's necessary to realize that this can't be college again...if weekends hanging out with friends or going out drinking is more important than having a weekend together or talking over skype, your priorities aren't set in the right place for a LDR. (Not saying you can't socialize at all, it should just take a back seat.)