Leftovers is good on any pokemon that you expect to be on the field, tanking hits, for more than four turns. There are not many Pokemon like that, but when you consider that if you give your Pokemon protect, it only needs to be a tank for two turns; if you give your Pokemon Helping Hand, it probably doesn't even need to tank anything; and if you give it bulk investment instead of offense investment, it doesn't care about a power boosting item;
then you realize I'm talking about Politoed, the only thing I've ever liked running Leftovers on.
Politoed @ Leftovers
252 HP / 252 Def / 4 SpD
This guy easily survives 7-8 turns when I need him to, packs a punch when i need him to (on neutral water types only), supports his mates when i need him to... Toed is a great supporter. And Leftovers is of more use on him than Sitrus because he's taking continual slow damage, instead of tanking one or two hits. I guess the relationship could best be summarized in "Sitrus for Tanks, Lefties for Walls;" and though there aren't many walls left in a doubles meta, those that are there still love this item.