I think it works pretty well at the beginning of the season; basically the fatigue model forces you to use a rotation.
It breaks down, in my opinion, as the season goes on. By the latter third of the season, you can start using ever shorter rotations, and in the extreme case, at the end of the season a guy can start 5+ games in a row assuming he has enough innings "banked." (Or more precisely, enough "pitches" banked.)
At least they fixed the post-season fatigue model; when I first started playing any unused IP carried over to the post-season. So you ended up with teams saving a guy like the 25 IP Milacki all season, not using him at all, and then running him out for 5-6 starts in the playoffs. Now, even if a pitcher has lots of unused innings (pitches), they treat him in the post-season as though he had used 95% of his availability.