I like tiamat on graves, ult into enemy team, team loses half health in one blast.. everyone runs.

It's actually not that bad on Alistar. Gives even better farming and some handy regen, but since I have philo stone on him and am usually free farming anyways, it's not nearly as useful as mitigation.

There are a lot of champions that tiamat is useful on, it's just unfortunately based on the splash radius which is too small, and stackign them takes up too many inventory slots. There are situations whent his can be a great item, like for a twitch pay n spray or sion ult or gp's parrlay, but for the most part this item is on the back burner because of its small radius, and that it gives regen rather than AS or crit chance/damage, which would be more useful for most AD.