So A LOT of people are against it, but just as many people are for it.

Personally, I'm for it, as long as both (or all) of the people involved are single.
To be completely honest I've got a couple as it is.
But it shouldn't be something like "Oh boy, lets take our friendship to another level, and instead of talking and hanging out, we'll just have sex and talk every now and then and have sex over and over again".
It should be something as casual as hanging out, or playing videogames with eachother, or talking.
Once you reach that point, you'll realize how great Friends with Benefits is.