My best experience with free agent was back to one of previous several seasons when I got a ML/AML with dribbler SA, and he was an extremely fast trainer. I got this guy and power trained him and gave him MC role; used him for several seasons. He could always score amazing goals per the comments, but unfortunately he was not a match decider so eventually I sold him.

Another funny experience was a ST wth one on one SA; I got him and sold him on market, I watched the auction on him and the auction went for 50 plus rounds :-), that made my day that day.