Lucina: I like her as a unit, otherwise fairly boring, has some good moments though.
Say'Ri: Never used her besides as a speed boost pair up for one of my units in a challenge run.
Gaius: Only use him to pair up with Tharja and pass down pegasus.
Vaike: Rarely use him ever and he is annoying.
Panne: Was useful for a little teeny bit in the same challenge run I used Say'Ri for, didn't stay useful.
Donnel: Loved him in my first playthrough, later decided he wasn't all that good besides good stat growths early on and mentioned earlier: His pot on his head.
Ricken: You are short and always will be, so shut up about it, only thing this kid is useful for is making a good sage Owain.
Lissa: Only good for making babies. (Owain) She starts out as an only-staff user and only-staff users are a pain to train early on therefore they grow weak and I stop using them so by the time it is easier to train them (Rescue staves) they are horrendously underleveled and I just don't feel like putting effort into them, sure they make nice sages but... meh...
Maribelle: Same reason as for Lissa, but she is a tad more useful because she is a troubadour which are generally superior to clerics in every way.
Cynthia: Rarely ever use her.
Emmeryn: I want to use her, but I know she won't turn out to anything more than a better lissa... And I don't like lissa.
Units I've never recruited:
Brady: I have often unlocked his chapter and got to the preparation menu once or twice, didn't feel like recruiting him/training Maribelle. He looks cool though :/
Severa: Don't use cynthia, don't want to use cynthia, too lazy to build support with cynthia.
Yarne: Don't use panne, don't want to use panne, too lazy to build support with panne.