These Acetal picks are great. (Delrin, Celcon, etc., are brand names)  Very smooth surface, durable and flexible.  Not as surface-hard as cellulose picks. They yield nicely to light acoustic guitar strings, and heavier string attack isn't compromised on accents. Perhaps a bit brighter sounding compared to same thickness Nylon picks, everything else remaining the same. Much depends though on how you play. Highly recommended.