It's been doomed the two times I've done it. One was my highschool sweetheart, her and I wanted vastly different things after school, broke up, kept sleeping together, got technically back together and broke up again a week later. That one was really done and we tried to make it work again.
The other, I dated this girl for a couple months, no part of it outside of the sex was particularly good for us, relationship was basically built on lots of sloppy drunken f-ing. Knew it wasn't gonna work but wasn't (And probably wouldn't if she knocked on my door right now) going to kick her fine ass outta bed so I rode that one till the wheels fell off.
Your mileage may vary greatly but generally you break up with someone for a reason. I'm watching a good buddy of mine do this right now. His situation was basically he was a drunken prick and really immature about things and she got tired of it after a while. He'd admit all the time that he wanted her back, he knew he screwed up etc... but he's not lost any of the bad habits he picked up in the few months they were apart so I think that for him as well, it's destined for failure.