You'll probably fall asleep earlier in the day, but you'll only sleep for 8 hours. This is why it's useful for offsetting your rhythm.
Ritalin/Adderall is very different. If you use it to stay up for 8 hours, you'll sleep for 16 hours the next time you pass out.
Again, though, the data is ambiguous AFAIK.
From my subjective experience, this isn't necessarily true. All-nighters with modafinil vs. all-nighters with adderall require the same amount of catch-up sleep for me. Modafinil just gives me a less jittery wakefulness.
I tend to prefer to use it to offset sleep for relatively short periods - e.g. get an extra night of work, and then sleep a bit longer for the next couple of nights - as I feel it's more productive to get a longer block of time where I'm fully alert, and give up late evenings when I'd feel tired anyway to recover.