I run daily, and I found that the first time I did it on modafinil it was awful...chest pains, general discomfort. However, I later realised that this was simply because it was my first time taking modafinil! Now that I use it fairly regularly, I actually find it enhances exercise, in that you take longer to feel fatigued. In line with the effects of the drug, it seems to be more a case of "not noticing" the tiredness, as opposed to, say, amphetamines, which actually seem to make you temporarily faster/stronger (by the way I don't recommend exercising on speed).
There has actually been a lot of controversy recently with top athletes having tested positively for modafinil. If pros are using it, it seems like fairly conclusive proof that it enhances your abilities.
In summary, and bearing in mind this is just my experience: it's fine, but only if you are fit and used to the drug, otherwise be careful (and generally just be careful exercising on any stimulant).