Having fitted a couple of designer rads in my house I would say;
Function - Disappointing
Price - Uncompetitive
Form - Lovely!
I'd more or less agree with that. A lot of 'designer' rads appear to be only single panel, so if you're accustomed to the sort of heat output you get from a double panel rad, you'll be disappointed. However, if you compare them with a standard single panel radiator, the efficiency isn't all that much lower.
For example, I have an 1800x600 vertical in the hallway, and the same radiator horizontally in the extension hallway. They are more or less the same as the Oceanus designer radiators sold by
Screwfix although branded under a different name. Both of these have more than enough output for the areas they are heating, as they were sized accordingly.
However, compare that to my bedroom where I have the same style of rad albeit something like 1000x600 in single panel, and it doesn't provide enough heat to warm the room to a comfortable temperature on a particularly cold day. I was always worried this would be the case when I fitted it, as the room has a large patio window and three external walls, although they are all insulated. Unfortunately space constraints meant that the radiator I fitted was the largest I could manage, and for some reason, the one particular size I wanted wasn't manufactured in double panel in the style/colour I wanted.
Conversely, I have been very impressed with an Ultraheat Trojan that I've fitted in another room. It's very reasonably priced for a designer rad if you shop around, and the output is absolutely excellent for its size.
So in summary, it's horses for courses. If you're particularly concerned about aesthetics then designer products aren't a waste of money, but if you're only concerned about function, you wouldn't be considering buying them in the first place. I think my best advice would be to shop around on the internet, as you'll find many similar products branded under different names and sold for varying prices. I managed to save £20-£30 per rad over
Screwfix prices on the Oceanus rads, and that was including delivery. I also managed to get a great deal on the interwebs when I fitted CH in my previous house with column rads in every room.