I've had a fair bit of experience (though not every day for months on end). From what I've read many people have a similar time, though some may end up with better results.
If you're tired, you can use it as an alertness aid. If before, you would have fallen asleep at your desk from exhaustion, you will instead be able to continue to work (but still be fairly tired). Modafinil will allow you to continue on from the point of exhaustion and be able to function in some reduced capacity (50-60%). You will experience some periods of higher function (almost like waking up and being a bit tired, and then your mind clears a bit), basically you will alternate between tired and fine for a few hours, each period about an hour in duration. Taking modafinil in this manner isn’t especially recommended, but if you just spent all day / night putting out fires and you need to come in the next day to ensure everything is still running smoothly (but hard work isn’t expected), modafinil will do great here. Alternatively, I would recommend using it at the beginning of a normal day after a full night’s rest. This will result in some periods of clear headedness, and generally make it incredibly easy to focus on a task. I wouldn’t say I’m as ‘quick’ on modafinil, but I can certainly refactor code and slowly improve it for hours without feeling bored or getting distracted. I find that it makes me super calm as well. It definitely hampers creativity / big picture thinking, I wouldn’t do system design on modafinil, or even design for a subcomponent, I wouldn’t be at my best and might miss details or produce a subpar design. Basically, if you are just implementing a chunk of code, and have already figured out the details, modafinil will result in a good experience, you’ll make steady progress, your work will be of high quality, and the day will go by fairly quickly.
aaron695 mentioned dose – 2-3 pills a week is fine. Alternating entire weeks on and off is fine. I would only take at most one pill a day, but some people take two and are fine. Tolerance builds up about as quickly as it does for Adderall IR. Definitely don’t go drinking on the drug, but if you typically have a drink with dinner you’ll be fine - just don’t expect to feel any of the effects from the alcohol. I’ve felt the beginnings of depression on other nootropics (specifically piracetam - I generally have a poor diet though, so that may have been my fault) – but haven’t had a problem with modafinil after an extended period of time. It will be difficult to sleep if you take it too late in the day, definitely dose in the morning.
Essentially it’s a stronger, better version of caffeine (increased alertness, focus, can be used to increase performance with acute use, but with regular use tolerance builds and productivity decreases).