First a warning, because I've seen many misconceptions: Modafinil is not a sleep replacement. Especially not long-term. Do not use it that way. Its anti-sleep property is that it suppresses many parts of the urge to sleep. It does not provide any of the effects sleep provides.
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Well, I've always had issues with sleep: I have an 25 hour circadian rythm, so before modafinil I often just let it run and wake up an hour later every day. I thought modafinil might be something useful to help me experiment with my sleep. Note that I did not get a doctor's receipt, so I imported the drug myself. (I'm not in the US; it's not a felony where I am.)
You say "in comparison"-- I must say that I do not have much to compare it to. The only other substance of this kind that I feel an effect of is caffeine, and it's on a completely different level, in all ways.
Side-effects I noticed were mostly dry/chapped lips, reduced appetite and diarrhea-- the last of which seems not to be very common.
In terms of its nootropic effects, I'm normally very bad at focusing on stuff for long periods of time, and for some periods it helped me focus for long times on tedious things. Or it helped me focus on cleaning the house really well, reading the internet thoroughly and avoiding what I had to get done. But if I used it and spent a night studying, I would remember the material studied very well. This concentration effect was also the quickest to fade after longer use.
The main effect is its anti-sleep property is that it suppresses many parts of the urge to sleep. It does not provide any of the effects sleep provides.
Right now I don't use it, because I am not working on anything that would benefit from it, and the side effects are not worth it to me right now. Also, melatonin has helped me with my sleep rythm issues for some time, so I don't need it for that.
I did not notice any addictive effects whatsoever. There's no high you get from it, nor a low after you stop, perhaps besides sleeping a bit longer for a few days.