The boost to alertness and general stimulatory properties more than compensate. Learning is improved over baseline (in non extreme sleep deprivation scenarios). Of course, some memory effects are easier to study than others. I don't recall what impact it has on skill acquisition.

Something I am more wary of is that forgetting may be impaired. The process of filtering through memories, sifting out the useful abstractions and discarding the crap is a fairly important part of sleep. I love having extremely retentive memory but I don't necessarily think it is healthy!

On the health side of things, taking Modafinil and cutting down on sleep doesn't make you sick on it's own if you are otherwise healthy. But it isn't without physical costs. Many users (including myself) note that you need to be in far more in touch with your own physiology. If you've turned the safety mechanism off and don't take care to monitor your health and force sleep when you need it you will wear yourself down over a couple of weeks. This means that you will be more vulnerable to the effects of stress and less able to recover from intense exercise.