I've been taking 200mg of Modafinil off and on for a few months. I immediately noticed a huge burst in clarity, ability to focus, cognitive capacity, energy and motivation. However these effects slowly faded and even after a month break, modafinil just leaves me feeling stimulated. It is worth noting that with this stimulation comes a certain freedom from mental fatigue. So if you're taking a long test you won't get that feeling like you just can't think anymore. In that respect it is nice for days that I haven't been able to get the sleep I needed and is nice for mentally intensive days.
Negative effects include an immensely decreased appetite, which although it can be nice to not have to eat as much, if you don't force down food you'll feel like hell by the end of your day. Initially when I started taking it I struggled to be patient with others, but that eventually goes away.