I agree with you. We hardly get any warmth through the van's heater by the time we get to work. In summer the engine warms up much quicker, but we want hot water in winter.
The new diesel engines are much more efficient and produce less heat. Range Rover has a Webasto diesel heater to supplement the heat needed to warm the inside cabin, so having one of these fitted would be a waste of time on that vehicle.
They're a great invention and application.
Our three Peugeot 406 hdi have auxiliary heaters fitted as standard kit, as do many of the PSA Diesels, and they start producing cabin heat pretty much immediately. The morning school run in the 2.2 coupe is about 6 miles and by then the water temperature is up to normal - however - it takes another 5 miles of the return journey to get the oil temperature up to normal. My old IDI Ranger is fully warmed up in around 3 miles.