Display List

using display list seems to be very good for gaining performance. But in which case it is useful ?
When an object is repeated a lot of times in the scene or when the scene is composed by a single object with lots of face. (or both?)

Display lists are good for static geometry. If you have an object that is repeated lots, then yes, use display lists for them. However, I believe on T&L cards display lists are stored in VRAM (Might be wrong on this), and soo excessive use of them might lead to severely reduced space for your textures.

Small frequent objects are ideal candidates.