They are a radiator, so in terms of efficiency they convert heat from liquid into the surrounding air. There are many low water content rads out there, the Jaga design is but one.
So technically nothing special, but if you like the look/design then they may be worth the cost to you.
I have installed around 20 of these units a few years ago, there is nothing wrong with them, just expensive, as you point out. They aren't usually in stock in the UK, I recall six weeks being the lead time.
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