you will like it 
i converted my fan several years ago to automatic operation using the switch from an old dehumidifier
but if i would have known about the Arduino then i would have used it as i found that summer and winter have an effect on the readings so i have to make it more sensitive in the winter and less in the summer
but with an Arduino you could just throw a temp sensor in the attic or maybe even just watch for a sudden change in humidity rather then above or below a set value so it wouldn't be effected by the gradual changes from weather/seasons
but i love it being automatic and i think they should make all of them like that so you don't have to worry about the super high humidity from someone taking a long hot shower and not turning on the fan