Modafinil is an extremely gentle stim, less dangerous than caffeine IMO. It's neither a potent releasing agent like amphetamine, nor is it a potent NDRI like Coke, Ritalin or the Pyrrolidinophenones. Its mode of action hasn't been fully elucidated, but it seems to have a very mild effect on dopamine reuptake, so it may actually be used as an antidepressant in patients who do not respond to typical antidepressants. Really, "wakefulness promoting agent" is probably a more apt description of Modafinil than "stimulant".
Though I'd avise you to always consult with a medical professional, I doubt much harm will come to you if you take it within the recommended dose range (i.e. between 100 and 300 mg per day, preferably starting with 2 doses of 50 mg).
That said, if you're bipolar you might still want to stay the hell away from this stuff, as staying awake for too long might cause a manic switch... also, if you have an addictive personality, I'd advise you to exercise caution as it is possible to catch an ever-so-slight buzz from Modafinil.