Once upon the time, he was considered a good character, because he is a story-farmable 2.5x rainbow captain. Of course, even then most people preferred to use Mihawk, but in the early OPTC days, he was the second 2.5x rainbow option. Since then, of course, he has lapsed to near obscurity, as characters such as Koala Revolutionary Army or DeCalvan Brothers Brothers’ Bond provide almost as much of a boost, and have much better stats and specials. And of course Knight of the Sea Jinbe Ex-Royal Seven Warlords of the Sea makes him hide in the small corner and whimper. For DEX teams, his special is also replacable by more versatile Izo's. In the end - yes, he can clear some content and help as a sub, but you will almost certainly have better units after few months of playing, and he will gather dust. Just like in the anime, Zeff is a relic of the past.
Final note: he is also useful in the rare occasions we get Rankings which reward you for completing missions with weakest (lowest cost) teams. Those are usually relatively easy, and he is the cheapest 2.5x ATK captain. Really good for getting top score in ranking like 'clear 15 stamina Kuro's Master Plan with team with lowest possible cost'.