I take modafinil five days a week on prescription for fibromyalgia fatigue. There's no withdrawal to speak of, but on my first "day off" for the week I do tend to sleep all day. I've never been sure if that's because of the modafinil or if it's just rebound fatigue from pushing myself too much during the week.
It does help a bit with concentration, but much, much less than when I first started taking it. I think these effects wear off pretty quickly. The only real effect is wakefulness, which in my case means less fatigue. It doesn't make you feel speedy or impact your mood.