I have used the yates/fifi for years.
On hard aid, I use 2 regular daisys and then "float" as Adam mentions the Yates/fifi.
Awesome for transitions, and when you have something really steep you can float it up to the piece and do a waist bounce test.
Also very smooth for when you are in desperate need to get into something, you don't have to use your ladder and regular daisy, you can just fifi in and pull tight.
For easy walls, I just use 2 Yates and try and never pull them tight on lead, it helps to have a regular fifi on the upper tie in loop for reducing the overall length.
Learning how to stand up against your fifi when in your ladders is the #1 skill most people never learn. It is full body tension and can eliminate many placements over the course of a route.
Practice getting as high as possible in your ladders, and then as you go higher, release some tension from the yates/fifi until you are upright and well above what would normally be possible.
this is where the Yates/fifi combo really shines.