For just text notes is very low token usage, i use Opus for everything and a whole week of notes is like 30m of a coding session

Not much, the main cost is having Claude write notes as we work, which isn't too different from what it does anyway with Plan mode, and it helps me onboard new sessions more easily. It also may save me some tokens because the tool I built helps it semantically search for relevant notes instead of wasting tool calls on grep and reading irrelevant documents.