Use Memory if you use ChatGPT daily for ongoing projects (a multi-month book, a long-running codebase, a thesis). Saved memories plus chat-history reference cut prompt-setup time meaningfully, often a minute or two per session. Audit monthly with the list-everything prompt.
Use Custom Instructions instead if the thing you want ChatGPT to know is stable (your role, preferred tone, default coding language). Static fields beat dynamic memory for stable identity because they cannot be silently overwritten by an inline 'remember that'.
Use Projects instead if the work belongs to a tight scope you do not want bleeding into other chats (client work, sensitive research, a thesis). Projects keep memory and files contained, with their own per-Project instructions.
Use Temporary Chat (NOT Memory) if the question is personal, sensitive, or one-off. Anything you research in Temporary Chat is excluded from Memory by design. Reserve regular chats for work you want ChatGPT learning from.
Disable Memory entirely if you share an OpenAI account, work in a regulated environment where every saved fact is a small audit liability, or simply prefer ChatGPT to start fresh every session. Disabling both toggles costs nothing on the answer-quality side; Custom Instructions still applies.
Our overall take: Memory is a net positive for daily users once you adopt the monthly audit habit. The two failure modes (stale facts shaping responses, occasional cross-project pattern matching) are mitigated by curating the saved-memory list and using Temporary Chat for off-topic queries. The chat-history reference layer is more useful than the saved-memory layer for most people; saved memories are most useful for hard constraints (allergies, accessibility, tooling defaults) that you do not want ChatGPT to forget.