I had fun recently. I am a member of Canon's CPS program- Canon Professional Service.
This entitles me to try almost anything Canon makes for 10 days.
So, I thought I'd try the EF 200-400mm f/4L IS USM Extender 1.4x.
The lens is a beast. It is very sharp but that comes with some negatives- size and price.
Weighs almost 8 lbs and cost $10,000 For more info on lens: