I have taken quite a few pre-workout supplements but I've narrowed it down to Animal Pack and Jack'd (which contains a small amount of creatine). Animal Pack contains 11 pills so its bit of an excersice itself, Jack'd is extremely tolerable though. Any one of these choices will significantly improve your overall workout performance. I bench press 240 normally but with these supplements i can push 265, to one rep of 280 and I only weigh 190(It gives a great amount of energy). Take fish oil and b12 complex, multi's aren't really needed if you go with animal pack, or the 2 listed. Make sure you eat an hour before your work out,take any supplement, stretch and work out.
My friends have said similar things, but I truly wonder how much of the noticed increase is due to placebo coupled with the energy boost you get from the one universal ingredient of all pre-workout supplements: caffeine.
I notice that if I go to the gym and, after my first set or two, "feel" like I'm going to have a good day, then I have a good day. If I feel tired, worn down, or slightly disappointed in my first set or two, unless I make a concerted effort to focus and truly expend maximal effort, my future sets will suffer and I'll have a less-than-stellar workout.
Given the aforementioned caffeine content in most of the supplements, I can also see how not taking it one day would make the user feel scattered, weak and lethargic--those being some of the effects of mild caffeine withdrawal.