I use one. It's a great tool for training, and it works well especially when on hills. I know that if I am at 173 bpm, I am about to max out and better not push further as I cannot sustain above that level. I cannot reach that range on the flats though, not sure why that is. I disagree with the notion to ride slow to go fast. I train hard in the limited time I have, and when I manage to ride with others I am always in the lead group. Going slow does not help my fitness level, and I am not concerned with the amount of fat calories I burn. I suppose if you ride everyday then recovery rides would be essential, but that is not my case. Not to mention, I mentally can't go slow.