I've been taking ~50mg modafinil 5 times a week for years. Last batch I bought was 2 years ago on the grey market (an online indian pharmacy) in 200mg pills form (so I divide them in 4 for a daily dose).

I don't take any on weekends or on holiday, and sometimes I take 100mg if I'm going for a long car journey or something else that requires complete attention...

The effects are very subtle but noticeable: more alertness and concentration, and a more positive outlook on things. I found it's also pretty good at keeping depression at bay.