I'm still using Flame Lash, myself. Molten Whip can hit hard if you combo it right (chains/Cauterise/FoO -> Engulfing -> Embers -> Molten Whip is an easy combo to build 3/3 stacks on Molten Whip), but I feel as if it tries too hard to be for something that DK just isn't, something more burst-y like NB or Sorc.
I feel as if Flame Lash is just better suited to DK, as it fits perfectly in DK's combo (Engulfing -> Fossilize -> Flame Lash -> Leap -> Power Lash). It's snappy, it's always there when you land Flame Lash after a Fossilize, and it doesn't require you to fit the combo around the proc.
Molten Whip, on the other hand, does require you to fit the combo around the proc (chains/Cauterise/FoO -> Engulfing -> Embers -> Fossilize -> Leap -> Molten Whip), has a longer combo (more likelihood of your opponent countering you), has a longer setup (more likely to screw up the setup), and requires you to redo half the combo if it doesn't kill your opponent (Power Lash only needs off balance applied, so you can get 2-3 Power Lash procs out of the 1 Fossilize).
So yeah, Molten Whip can hit very hard, but I just prefer Flame Lash.