This card is a fantastic deck-cutter.
As skill cards go, +1 icon on a single test is low, the variety helps a bit, most characters can muster a use for either or , and anybody can use .
The benefit, the mini tutor, is just crazy for an action-less and cost-less effect. Use it to pull out key cards a little faster and/or obtain solutions to standing issues.
The low boost and useful success benefit means that you should play Eureka! on low risk tests, use it to push for a +3 or +4 advantage to secure the test. On standard/easy pushing for +2 is fine in early scenarios.
Any character with innate access should give Eureka! at least a passing thought.
Minh Thi Phan and Silas Marsh really like skills, this is obviously true for a strong one like Eureka!.
Eureka! is uniquely good for Norman Withers (for whom skills are low priority) since it bypasses his revealed top card and shuffles his deck, this gives him two chances to dig useful cards out of a deck with something unhelpful revealed on top.