Against - from what I've seen a large majority of people in online dating sites are, in truth, just looking for a quick hookup. The ones who aren't usually are on the more expensive sites. Another disadvantage is the safety factor - although that can be worked around by using a safe call (telling someone you trust who you will be with, where you will be, and arranging to call them at a specific time and use a code phrase to let them know you're okay) and meeting in a public place.
Advantage wise, it's a great way to meet people if you're busy. Outside of work, church, and social engagements like the kids soccer game, it's hard to meet someone unless you go to a bar or a party someone is throwing. If you don't want to deal with that scene, the internet is a great way to go around it. It's also great for people who are willing to move to possibly find their 'soul mate', even if s/he doesn't live in their area. I know of two successful marriages that started online, and one successful relationship.