Being rather tricky to remove outside of combat made Fiendslayer Paladin rather valuable in Standard. He was a great piece for White Weenie decks and his lifelink made him a great ally with the best mythic rare in Magic 2014, Archangel of Thune. He was an easy pick in Limited that also saw Standard play. However, he doesn't stack up to his competition once you get into the Eternal formats such as Modern. Then again, there have been Legacy Death and Taxes lists that have played him, so there is that.
However, like many playable Standard cards, Fiendslayer Paladin has found a few homes in the EDH format. Odric, Lunarch Marshal has been especially kind to him. With the Lunarch Marshal on the board, Fiendslayer Paladin allows all of your creatures to gain first strike and lifelink. Fiendslayer Paladin also is at home in Human Tribal Sigardia, Heron's Grace decks.
Fiendslayer Paladin may never be much more than a $1 rare card when it comes to price, at least in non-foil. However, Fiendslayer Paladin foils have been a consistent $4 for years. While this is a card that's unlikely to be a big investment long-term, he's a solid card to hold onto if you want to stick him in a Death and Taxes deck or in an EDH deck. Fiendslayer Paladin is a card worth holding onto.