I have had mixed results. The first time I took it, I couldn't sleep for like a day. Other times I was just pretty alert for awhile. Other times it didn't really have a very noticeable effect. I think it has diminishing returns....and I don't use it often at all so it isn't like a big tolerance issue, I don't think anyway.
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I'm "kind of a big deal".
http://img.imgcake.com/cyclo/Cyclopngegpngre.png