Any relationship that you want to last, should be taken seriously.
Will others take it seriously? In this day and age, it's a lot more accepted, but you'll still get the odd person like Avarance who doesn't understand the concept of modern life.
It takes a lot more than your average relationship, to make it work, but one could argue that it makes you value the time together even more so. One thing's for certain, it takes 10 times more trust than a regular relationship.
I think "online" relationships are bad, as in, ones that stay online with no plans to meet or progress. Meeting online, and using it as a main source of communication in between meeting, that's fine.