First off I would like to say that everyone reacts differently to each supplement. Personally, I have taken Jack3d and I thought it was alright, except the taste was a little hard to get used to. I did make some gains while on this supplement, but I prefer BSN No Xplode Fruit Punch. I didn't have that jittery feeling with this supplement.
If you're looking for a natural alternative, I've read that eating oatmeal 15-45 mins before working out can really help boost your energy and it provides the fuel to make your workouts more intense. If you want you could try the oatmeal as pre-workout meal and take Controlled Labs Purple Wraath before/during/after workouts. You can read about it here:
http://www.bodybuilding.com/store/clabs/purple.htmlBest of luck
S.A.47