Honestly, the starter 7*'s are only semi-useful. They're fucking amazing if you're new and can use them as friends, but otherwise they're kind of niche. Eze, Lance and Vargas are pretty much useless. Selena is a solo monster (use her for mecha farming). Atro and Magress are basically useful for some of the harder content that we don't have yet, but only like 1 battle each.
Overall they have ridiculous stats, but right now there's not a whole lot that require them. I've got Selena up as my leader just for the novelty factor.
That said, the OTHER 7*'s are fucking ridiculous. Especially Kira's batch. Feeva, Kira, and Kanon are all monsters at what they do. Torah is also pretty powerful if you need a status inflicting unit, and Taser is the best STBB unit by a mile. Alpha is the only weak one of the batch, but is still better than the starters (minus Selena, because the revive and heal is just OP as hell). As you go further into the 7* batches, the abilities get more and more ridiculous, to the point where I don't use the starters for anything anymore (exception being Selena for mecha's).
It's unfortunate though, because JP has fallen into the same problem that global has had for a while now; the units are seriously outpacing content. This is mostly due to Gumi (and therefore Alim) having financial difficulties. I've got Kurt 7* but I don't have the time/patience to level him, because there's really no use. With Global releasing Kira's batch the day after the starters, it's really just stupid. All in all, I think Brave Frontier is probably in it's last year of viability.