Just returned from nine nights (single Dad & two kids, 11 & 5) and we all loved it. I've been to Fiji a couple of times previously but this was the first time out to an island and it was definitely worth the extra expense.
Water activities were excellent, in particular the water jungle and even the sand-bar snorkelling was good for kids. Food was pretty good, though it was difficult to get booked into anything other than the main buffet rom for evening meal, but we did go to Tavola twice.
I'd probably only do 5-nights next time and go somewhere else too, but I would definitely return here.