like : - what is the reputation of plpython for a dba?
Dunno.
- are there actual production system that use it
I used it in an SMS processing system and it worked like a charm. That system never made it to the production phase, though.
- what would be the recommended usage perimeter ?
(only administration script like function, advanced processing, etc ...)
I would say, in general, use untrusted languages only if the trusted ones are not capable or practical for your goals. (Reading/writing/processing data outside of the database, using third party tools for processing, etc.)