I tried to make extended properties work for a year but found them too much to manage with all the change scripts I generate day-to-day. Eventually I reworked them as a few tables as a data dictionary which was much simpler.

Storing descriptions seems the least useful reason for extended properties. Attaching attributes to tables such as "inserts, never updates" or "sync up and down", attributes used by development and deployment tools seems much more useful because they can be integrated as code.

I'd also see advantages to storing "edit masks" and "display formats" and other GUI attributes as column extended properties. Then, using those in code generation solutions.