Use it for short burst sprints more than sustained effort... Coupling it with a high-effort strength movement is really beneficial (i.e. - heavy set of squats or pulls, 30s rest, followed by 30s max effort ski erg sprint)
while the ski erg is great i will say it's my least favorite of all the similar equipment. if you have access to one it's great for switching things up, but imo no one really *needs* one