I've found the same thing as far as taking clen before workouts. The shakiness, heart pounding and general feeling of malaise would more or less ruin my workout, so I too decided to take it at the end of my workouts. I do like it before low-medium intensity cardio sessions however, but I think performing HIIT cardio after dosing clen would be a painful experience literally feeling like your heart was going to jump out of your chest.
In myself, it tends to really rip the water from my body, and cause a very flat look due to loss of muscle glycogen. I tend to lose a rep or 2 on exercises per week when using clen, so I've been somewhat hesitant to use it in cutting cycles as a result. The loss of reps and flattened look always makes me nervous about losing lean tissue.