I bought a pair, installed them in the derailleur I was using and rode on. Problem solved.
A few years later, the first derailleur was damaged and I moved the to another derailleur on a different bike. Time passed. I rode a lot of km, made many tours, including Land's End to John o'Groats with these jockey wheels spinning behind me the entire time. Everything worked and I forgot about them.
Last year, the derailleur on that bike also became damaged and I replaced it. When I took the old derailleur off, I found my tacx jockey wheels again. They didn't fit in the more modern replacement derailleur (new ones do), but as they still seemed OK I kept them in case they were useful later. This week I decided to take a closer look at them:
One has a little play in the bearing, which makes a bit of a gritty noise. The other still seems to be perfect. They still spin better than standard wheels in almost any new derailleur. The worn look on the outside is largely due to the teeth having worn away. However, teeth on jockey wheels aren't really that important. Some old derailleurs had round jockey wheels without teeth, but they still worked as derailleurs.
Needless to say, I'm impressed by the longevity. Mine were the lower cost type with the normal bearings. Now there are variations with stainless steel and ceramic bearings. Good quality products like this are exactly the sort of thing we set our shop up to sell. We don't like gimmicks. We like products which are reliable and long lasting.
If you've got worn jockey wheels in a derailleur, it's a lot cheaper to replace just the wheels than the entire derailleur. We now sell a selection of different types to fit most derailleurs.
I'm now testing ceramic Tacx jockey wheel in the Mango. While my old ones look odd with the teeth missing, I don't think it's that important. I've got one very old derailleur with round jockey wheels which never had any teeth at all.