Martin Logan electrostatic speakers, How do you know if the panels are good?

"Is the old, top of the line still good vs newer versions?"

Assuming a well cared for and proper functioning used set, the older Summit will unquestionably sound significantly better than a comparably priced new model, if and only if paired with an amplifier that can drive it to its potential and placed properly. 

I believe the panel reliability issue was due to the film coating process on older models.  Summit uses the same panel as Summit X and has the newer electrodeposition process.  There have been other improvements in later models, but I think panel reliability should not pose a problem. $3k seems like a good price if they are in good condition.  As a reference I sold my newer model Ethos for $3k also.  If I had the choice I would take the Summits over Ethos any day and definitely the Summit over brand new bottom of the line ESL ($2.5k) or ESL X ($4k)

I have a pair of ML Summits I have used for years. The panels were never an issue. I did have trouble with the woofers however and eventually ended up replacing all four and never had an issue since. ML is also great at supporting their customers. If you can get a pair of the Summits for $3K I would put that buy in the steal category.