I find I have even more energy and the ability to pull or push more weight than usual. I lift weights fasted most of the time, if I had eaten plenty prior that day I really struggle to reach my goal of improving my strength. I can lift noticably longer when I got food to burn, but I am not quite going for an improvement in my endurance.
If youre going for building strength, high weight low reps, fasted workouts are amazing. If youre looking to maintain and build endurance, eat prior to your workout.