I normally use 6-7 foot leaders. Why? Because I re-tie often, and the FG knot has really helped me to facilitate that. I could not use leaders for days at time with other popular adjoining knots because of knot failures due to guide fraying. Have I had FG knots fail? Yes, and I have figured out why, but I don't want to hijack my own thread by going off topic.
I use leaders for my treble lures, t-rig, and spinning gear applications. I enjoy using braid to mono, copoly, and fc leaders because I find they offer the best qualities of each line. I stay clear of leaders for power fishing applications. Hard hook sets + heavy gear is where I stop using leaders and transition to using just one line.