Not really, their drop rates are pretty horrible. You can slowly imp up a few units, but don't expect to max out a whole bunch. What makes it hilarious is on reddit etc. when people discuss units as if their imp stats are their base stats. "Her attack sucks but nothing some imps won't fix!" More like you'll probably never use imps on her, and her attack stat will suck.
I've seen one in the first 4 zones of Vrik, so they're sort of rare. I maxed Elza, am close to maxing Zelnite, 1/2 way maxed on Zelnite, and will start on Maxwell once I'm at that point. You get more imps from gifts, Frontier Hunts, and super honor summons.
Vael should be good once his SBB is fixed. Kushra will get a 7* form (I think?). Lilith is a good boss killer with an infinite-SBB. I have no idea on Zergal.
Two fights 3 maidens per fight. First round is hard, second is cake. Lots of status inflictors, bb gauge drain. Reward is a gem and a stat sphere that increases attack by 100%, crit damage by 50%, but reduces rec and def.
Those are some terrible pulls. I recommend not doing any of these single unit rate ups ever unless you're buying gems. Summon when it's player's choice or a batch (6, not these half ones they've been doing). Otherwise it's just too unlikely you'll get something good. Exceptions would be something like Tridon, who I absolutely used 10 summons for because he's so OP (Ultor and the rest of the SBS units though, pass).
It'll be better once they release RC4 and 5. You've got to keep in mind RC3 was meant for pre-Zelnite's batch. Gumi just fucked it up hardcore with releasing units so quickly and no content. Really, we may end up in a similar predicament with RC4 as well.
