At least as I've experienced it, modafinil (Provigil) is a very useful, effective stimulant with very few side effects. But remember, it's still a stimulant.
I recently started taking 100 mg tablets of modafinil (Modalert, a generic equivalent) for sleepiness brought on by (what else) not having enough time to sleep. I've found that usually, one tablet is sufficient for a significant effect in the reduction of fatigue. Of course, it's not perfect; more than once I've dozed off while on modafinil, although always in situations sufficiently boring that it's quite possible I would've dozed off anyway.
I find it difficult to qualify the feeling of being on modafinil. I don't have any experience at all with amphetamines, and thus can't provide a comparison there. I can say that subjectively, it provides much cleaner, smoother, less jittery stimulation than a pill of Vivarin. Its effects seem to differ widely according to my mood at the time of ingestion. The stimulation doesn't feel especially "pushy;" I've never had any trouble going to sleep, and I've never felt like I really had to do anything. Depending on my mindset, however, it can provide something like the restless energy caffeine gives me, or it can just leave me feeling clear-headed and unfatigued. Sometimes, it can be pretty euphoric, too, which kinda worries me.
The reinforcing properties of modafinil are, apparently, disputed. The packaging insert for modafinil notes that it has roughly the same euphoric and reinforcing potential as methylphenidate (Ritalin), while other studies show no addictive effect, except in animals/human patients previously exposed to cocaine. In my personal experience, there's definitely some reinforcement, although it remains unclear whether it acts upon the all-powerful reward pathway from the ventral tegmentum to the nucleus accumbens (the pathway implicated in the addictive properties of, among other substances, nicotine, cocaine, and amphetamines) or I simply like to take it because it's effective at keeping me alert.
As for actually increasing intelligence, one study places modafinil at about the level of four cups of coffee. Another study showed caffeine, modafinil, and dextroamphetamine (Dexedrine) all restored mental performance to the same degree. In non-sleep-deprived individuals, effects are apparently "limited to the span of immediate verbal recall and short-term visual recognition memory, which is insufficient for it to be considered as a cognitive enhancer in non-sleep-deprived individuals." Personally and anecdotally, I see no real effect on my intelligence that I could attribute to taking modafinil. Perhaps it provides a bit more focus, but it's not the glorious smart drug some have made it out to be.
No, the real reason to take modafinil, besides simply feeling awake, is its ability to reduce the need for rebound sleep. Patients receiving amphetamine or placebo deprived of sleep for 72 hours needed 15 hours of recovery sleep, whereas patients receiving modafinil needed only 10 hours.
Now, is this good for you? No one really knows. Although a few conjectures have been put forth, no one even knows how modafinil works in the first place. As for long-term safety, many narcoleptic patients have used modafinil daily for years without adverse effects, but narcoleptic sleep patterns aren't quite "normal," and narcoleptics use modafinil to stay awake during the day, not to decrease the amount of sleep they receive in total. Some people report anecdotally that extended use produces noticeable cognitive impairment, although there are no studies to back up these reports.
Although the exact purpose of sleep is still a mystery, it does seem pretty likely that it has some function that a drug can't take the place of. Come exam week, perhaps I'll find out for myself. | 
