I ran a HOB ecosystem for around 4 months. It semmed OK but I was not highly impressed. The siphon was always a problem because the flow rate was so low air bubbles would get trapped in the U-tube.
Also there was not a great deal of room in the HOB model for the caulerpa to grow.
IMO they are not worth the price. I have however read many possitive comments on the systems.
Read the post in the FO forum titled "HLLE Purple Tang update" for example.