I'm currently using Eirika only, but I do intend to use more.
Eirika was kind of my "test" DLC just to see what they could be capable of, and she is currently a very powerful unit (but that is to be expected with grinding, haha) and capable of soloing some of the later SpotPass. I think I leveled her through all the Sword-primary T1 classes, then as a General for Pavise, then a Swordmaster for Astra/Swordfaire, and currently an Assassin. I think I'll do Dark Flier for Galeforce next.
I plan to level other SpotPass units through similar routes, but yeah.