Of course it's successful if it wasn't why would they continue to fly into BOH year after year. My only wish is that it would become a year-round service perhaps not at the same level of frequency for the summer but at least twice weekly. For BOH to maintain 6 flights a week in the winter to GVA is a very good indicator of the potential the airport has for other route development.
HOWEVER if further expansion at EXT goes on this will be limited. More than a 3rd runway at LHR, expansion at EXT would take custom away from BOH. For example customers in Weymouth would easily switch between EXT or BOH depending upon air fare - it's about the same travel time given the awful roadworks/gridlock/traffic going on in Poole & Bournemouth.