I've been taking Modafinil @ 200-400mg/day off and on since 2003, with probably a total of 4 years of usage in that time span, together with Xyrem for about a year in 2004 and punctuated by ritalin for a year or so around 2008-09. I personally only take it when I have to and try to avoid taking it on days on which I think I can manage without it.
Yes, it can be habit forming, but only psychologically and only weekly. I can totally see it being habit forming when "taken together with" a demanding worklife. If you aren't working lots of hours where you need to stay focused the entire time, there is little motivation to take modafinil. It's not any "fun" and the "high" you get is not from the drug itself, but the ego boost from getting a lot of shit done. People don't get "addicted" to the drug but to the feeling of productivity.
On top of that, it has little to no side effects. It might have exacerbated my bruxism slightly but that's about it.
However there is one risk with it and that is sleep deprivation. My number one advice to anyone that takes it is to never take it after about 11am in the morning (or less than 12 hours before the time you intend to sleep). If you follow that one rule, I can't foresee people having any issues with it long term. The problem with taking it less than 12 hours before bedtime is that it has a half-life of about 6-8 hours meaning that it's mostly worn off by 12 hours and sleep "no longer feels optional at that point. This approach lets your feel awake, alert, productive and focused during your daylight hours when you plan to be working.
If however someone does break that rule, it's easy to lapse into a messed up sleep cycle and that itself can lead to someone feeling more and more tired the longer it goes on leading to trying to fix the problem with more modafinil.
The side effects cited from Internal Medicine (“hypersensitivity reactions and neuropsychiatric adverse effects”) aren't a product of modafinil per se, but a constant accumulated lack of sleep. Add to that the fact that the symptoms of sleep deprivation are pretty much indistinguishable from schizophrenia and you have an explanation from those side effects.
I doubt that abuse of this drug would do anything to impact my ability to get this medicine even if such abuse was widespread. It's not a trivial thing for someone to fake a diagnoses with narcolepsy, like you can do with ADD/ADHD or anxiety disorders. A bona fide diagnosis requires a polysomnogram and MSLT if you don't show any serious signs of apnea. If one were to try to fake it, they probably need advice from an actual narcoleptic like myself to know what to say and do to fake one or both of those tests.
Plus, modafinil isn't even the most restricted narcolepsy drug. Xyrem is, which is basically pharmaceutical grade GHB. That drug only comes from one pharmacy in the country and is overnighted in massive boxes so that it can't be trivially stolen. The boxes are huge for two tiny bottles. TBH the only thing that prevents narcoleptics from getting access to a drug they need like xyrem (a necessity if you have cataplexy) are drug costs. If you don't have group insurance or if narcolepsy/cataplexy are pre-existing conditions for your insurance, then Xyrem will set you back around $50k to $80k a year last time I checked. I don't know street prices for drugs, but I imagine that they are 100-200x more expensive than a legitimate prescription. The reason for the cost is the fact that it's an orphan drug, meaning that the costs of bringing it to market are spread across a very small potential target market. Even that's kind of absurd since it's sleep benefits were known from back in the 80s when GNC sold it OTC well before any clinical trials. FWIW Faking cataplexy is largely impossible if your doctor performs the blood test available to diagnose it.