Modafinil is indicated for three things: obstructive sleep apnea, shift-work sleep disorder, and narcolepsy. IME, it requires prior authorization.
The generic is around $40 per 200mg tablet. If you are finding it online and it's nowhere near this cost, it's likely some other nootropic.
How does it feel / work? You won't feel anything. It's not like adderall or any other amphetamine-based stimulant in any sense. You're just simply "awake." The mechanism for this is not really known. I was on it for a solid month before getting my sleep study and CPAP. It will not increase your brain power or any of that nonsense. Won't give you the "focus" or "energy" other stimulants do. You are just simply "awake." In my experience, it did not contribute anything towards learning or memorization--just solely the ability to stay awake.
If you do decide to take it and feel anything other than "awake" it is likely some other stimulant or nootropic. I (and many, many, many others that have used legit versions of it) can tell you this. I have no idea what all the forum nonsense is about out there on the internet, but I'm pretty sure what they got was not legit modafinil.
Anyways, should you decide to get it--don't drink alcohol with it or do any other liver-toxic drugs. And the one thing you absolutely HAVE to watch out for when taking it is rashes. It's a rare side effect, but if you get a rash stop taking it immediately. Your epidermis is probably starting to detach from your dermis and it's gnarly. Google stevens-johnson syndrome.
Finally, if you decide to take it. Start at 100mg in the AM. If this doesn't work for you dose 200mg in the AM. If you find yourself getting tired around noon (which I was, because you can't outdrug a serious problem called sleep apnea) dose another 100mg. Do not exceed 400mg in a 24hr period.
Hope this helps. I think a better option would be to check out nootropics like piracetam, oxiracetam (I like this one, provides a little bit of a stimulant effect and seems to increase my memory retention), and aniracetam. There's entire guides online of all the ways to do these proper, just google around and you'll find them.