Totally do it if you're certain of your feelings. Communication is key.
We've been ex's for 3 years. Went out for five years.
We still hangout every couple Sundays. Watch a movie or two and spoon, then off to bed. It rocks.
There was once, early on, when she told me she could fall in love but this was before the fb relationship was truly established. Now we have large fun. We know each other's sweet spots and how to get it done. It's totally emotionless other than the passion for sex itself.
I honestly think the key is that we talk about our lives. Who she's dating and vise versa. She asks me advice about dudes and I get reciprocated with b1tch info. It keeps our friendship strong. We laugh together about relationship problems either of us, or people in general, have to face and truly appreciate that we are free and void of the sh1t that goes with having a bf/gf while we hangout.
We could be a rare case but it's been totally worth it to me.