Just to throw my voice into the mix - my DG-300 was retrofitted with
(factory-supplied) winglets...

The difference in L/D is so small with most winglets, that even if its
working correctly it would be hard to notice... Like others have said
- its "almost as good as" an equal improvement in span. What's the L/
D difference between a good 15-meter ship and a good 16-meter ship?
Very little.

However, I do notice an improvement in slow-speed handling with my
winglets on. This is where I believe a lot of people derive benefit
from them: Improved airflow over the wing at slow speeds and while
turning can directly translate into better thermalling performance,
both in reduced drag and in improved aileron control/authority.

--Noel