I've known a couple of people for whom long distance relationships have worked... I've had experiences myself, none of which worked out, but I've certainly learned a lot.
I have seen that these things can most certainly succeed and create a loving and stable relationship but I've also seen the darker side of long distance relationships and know of the frustration and pain they can bring. While I myself would never cheat on a boyfriend, fiance, husband -- whatever title my partner would have -- I know the pain of hearing one you care for has broken your trust and your heart (whether due to cheating, lies or whatever).
It really is down to both people in a long distance relationship to try their hardest and make things work. Mistakes can happen (whether through fear or plain stupidity) but I find online it can be so much harder to forgive... partly because you can't honestly have that person look into your eyes and tell you that they are genuinely sorry... I do my darndest and have done my darndest to forgive those who have hurt me, it's in my nature but it can leave something rotten inside that may even leak into "in person" relationships and that, I feel, is the most dangerous part of long distance ones (especially ones that occur online).
That said... when things go well... I think there is a lot to be said for being in a relationship across a great distance -- of all the people that person could have chosen, they chose you
don't even live in the same state or country (whether temporarily or otherwise)...
They really are a double edged sword.