I put in Supavent last year, and I have another for the other end of the roof ( 2 are recommended for this size roof), but havent gotten around to installing it yet.
There is certainly some hot air going up out of the hole, but I've come to the conclusion that the whirly bit is more of a triumph of marketing over reality- the hot air was going up and out at about the same rate whether or not it is spinning. I reckon just as much air is being forced into one side of it as exits the other side when the wind makes it spin, so no real "suction" of extra hot air from the roof cavity.
So, I decided to modify it with some cut up pieces of 1.5l plastic mineral water bottles to make 3 makeshift blades that we glued to the spokes in an attempt to force more air upwards. It appears to make a slight improvement, as in the air does rise faster when it is spinning... noticeable, but not a huge improvement. We tested it indoors with feathers to measure updraughts

. I think that more significant modifications could be done to make it shift more air with better blades, but thats a long way down my to do list.
I didn't do any before and after temperature measurements in the roof space, so nothing quantitative, other than, yes some hot air was venting and cooler air was being drawn up through the access hole, so it would have lowered the temperature a bit. Once I get around to putting the other one up I may log some temperatures outside air temp from Wx station, vs roof space vs inside on says of similar weather... but thats also a way down the to do list... and depends a bit on how much the thermocouples for my data logger might cost.
I installed one on my power shed (garden shed from Bunnings), and that definitely made it cooler in there, but it is a much greater vent size proportionally, than what is on the house.
They look like they might allow rain in, but having seen it in a 240mm/hr downpour, I'm happy to say it doesnt leak. Also, mine are very quiet as they have decent bearings, we just dont hear the one on the house roof, no matter how fast it spins.
A fan would definitely improve its effectiveness, however, Its hard to know if the power used by the fan would be offset by less AC use in a building. Maybe one of the new low power ceiling fans would do the job nicely at minimal power cost. Actually a fan into a covered vent without the spinning bit would be better.
Is it energy efficient? for sure, it uses none!
Does it lower the bills? hard to say, as havent paid an electricity bill since 1991

I've just thought of another example of the whirly bit not doing anything useful- I have a smaller one on my composting toilet vent pipe. Sure it spins in a breeze, but didnt do much in the way of sucking up the fumes. Painting the PVC vent pipe black was a
lot more effective in drawing air upwards, but only worked when the sun was out, so I now have a 24VDC 3.6W fan running continuously there. Since it's venting the composting loo chamber, its a damn good test (with my nose) of how much air is being drawn upwards by the whirly bit... I reckon it is bugger all!