I've used it in two arenas with positive results.
Studying / Memorization - Find this works great for studying. I actually stopped taking my Rx Strattera and now only use Spike or Power Drive. I don't do this everyday though as I like the novelty of taking the Spike. I noticed increased benefits taking at the time of studying and at the time of the recall. So 1 study night and 1 at test time.
Additionally, I notice that 1 is better than 2 for studying.
Training / Competition - It is unbelievable for any type of training activity. I use it outside the weight room specifically on my runs. I can actually say that I noticed benefits beyond that of simply a better training experience. My paces are increased and my cadences are much more in tempoed. In fact I notice such a difference that I don't use it as much so that I can still get that nice jolt come competition time :).
I notice that 1 works great for training and lower intensity / longer duration activity. Long runs / Cycling
2 works great for short burst intense activity. Sprint Runs / Sprint Cycling
I keep it as a novelty item with specific times I use it. Another time I love to use it, is learning a new activity, specifically something more difficult as far as coordination is concerned. When I hit the pool on a day where I know I will be working on my stroke technique, I'll Spike up because I know that I am more in tune with my body. Definitely increases proprioception.