It had a huge effect for me. It seems for me at least that my HR would not rev up. For a while there it wouldn't go over 123 BPM. No matter how hard I tried, I would go anaerobic over 123. Prior to this my max. sustainable HR was 165. In playing around with my dosage I have gotten it back up, but can't seen to keep my HR there. It's very frustrating. In Oct I did The Tour de Safford 104 mile with 123 as my max HR. I guess you know it wasn't my fasted century. I went out for a while today to do some hill repeats and got it into the 140's only so I went home. It was to cold for me anyway.
http://connect.garmin.com/activity/61983434