I really notice it, but I've kept the dosing to once a week, when I really get the chance for a big cardio session. I find when I'd expect to get fatigued, it allows me to push around 30-50% longer. About equivalent to the difference between DMAA pre workout and a Diet Coke as far as total calorie burned (via heart rate monitor).
The caveat I'll add is that I've always done a lot of cardio- long bike rides, etc. If I'm not really pushing my limits as far as longer, faster sessions or training on short rest, I don't notice anything from the cardarine. Your mileage may vary, but it's at least producing the advertised effect.