I've been on modafinil for a number of years now (prescribed by a doctor in relation to a real medical condition). It is indeed a very interesting drug. There's a reason it's the brand names are Provigil and Nivigil. "Vigilant" is indeed a very good way to describe how it makes you feel. Like caffeine but cleaner (no jitters, etc.).

The downside is that you build up tolerance pretty quickly and that feeling dissipates. At this point, I really only notice if I don't take it (as you might with your daily cup of coffee). I honestly don't know if I continue to get on-going benefit, but I'm don't have any side effects, it may have some neuroprotective effects and getting off would be hard.