I think it's subtle. From my experience (!) if you take it and then don't really do anything then the drug appears negligible. However, if you take it and then turn your attention to a specific task the effects are quite noticeable, i.e. increased focus, obsessive concern with detail.
Turning to EDandmore, I tried the Spier modafinil from them. I didn't really notice much effect with it, but thinking back, I was probably expecting too much from it. I might give it another go