hello igneous. i'm writing from the tutukaka coast. the poor knights are my regular dive site. unless you are in to wreck diving, the poor knights are the place to go. as for operators, i use dive tutukaka, yukon and oceanblue adventures (liveaboard only).
i take the point that dive tutukaka can be a bit of a cattle boat, but so can yukon on busy days. all provide a good level of service in my opinion with each having minor advantages and disadvantages. if you can stretch to a two day liveaboard with oceanblue adventures, you will get a good experience of the poor knights.
as for cancellations, they can happen at any time of year. the poor knights are in open ocean in the wild south pacific, but as frosty says, april is usually a good time to visit.
lastly, the geography is definitely temperate/sub-tropical rather than tropical, a lot of tourists get caught out by this. the air and water temperature might not be as high as you think.
here's a short video i put together. it will give you a flavour of diving the poor knights and a few of the wrecks in the area.
[video=vimeo;122169997]https://vimeo.com/122169997[/video]