https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/232117828193

Are these any good

We had some on our 90 TD5

They do a little bit of de misting, they won’t clear the side windows but if you wipe the windows with a cloth first they do keep a portion of the window clear around the lower front part.
They work a bit better if you put the heater blower on full power but then it’s very noisy.

Not sure it’s worth the effort.

They have been around for yonks, some people like them whereas others say they are not worth the effort. I should imagine that as long as you are pushing a decent amount of air through the windscreen vents and the demisters seal correctly then they can't do any harm. Personally I made my own demisters which are hidden from view and fed directly from the large duct which forms the lower dash.

As the advert says, they are for the later wide vents (although I disagree with the date shown, that is when the 110 was introduced and not when the width of the vents changed).
My 1986 90 has the original narrow vents.

My stock set up ('95 defender 90) worked "ok", at full blast the front screen would just about clear, but never really the side windows. I installed the defender demisters (these look the same), and on balance I do think they work better, the screen clears more quickly and it does clear the side windows, at least a bit. So, based on my experience, I would say, yes, they were worth getting. In the end, the basic defender heater system is still a pretty "weak" system, and it's trying to demist a big drafty metal box, which just loves condensation, so don't expect miracles just by changing the way the air comes out!

It’s wierd how they vary as well, the blower air output on our 90 was fairly poor but on our 110 it’s very good.