We just completed our Advanced Course with the team at Nautilus and had a superb time.
The entire operation is extremely well-run from start to finish, meaning the focus can really be on enjoying your dives rather than having any safety concerns etc.
Our instructor Mike was great - thorough, patient and nothing was a problem.
Although we didn't dive with any of the other members of the Nautilus team, a number of them have been diving in Vanuatu for years and years and as a result they know the dive sites so well that every dive they seemed to have found all sorts of cool sea-life that the customers wouldn't otherwise have seen.
Would thoroughly recommend diving with these guys.