I think it depends largely on why the original breakup occurred. If it was infidelity, even if you can find it in yourself to forgive (truly forgive), then damaged trust has to be rebuilt over time, and the perpetrator has to take the lead role. If that doesn't happen or starts and the wavers, BAIL. That person hasn't accepted full responsibility and will do it again.
Likewise if the breakup happened because of too many arguments, and the *root* causes aren't addressed, then it's just a "comfort" reconciliation and won't last.
Sorry to dampen the mood, but notwithstanding my handle, in my experience they don't work.