Well, now that I've played it all, here is my review.
NightMist: Perhaps my favorite character. Lots of damage redirecting, card management, and jack-of-all-trades-ing. She can destroy environment and ongoings, deal LOTS of damage, heal a lot, can control the villain AND environment (or even hero decks) with the one card that's basically infrared eyepiece, she can draw cards (and have others draw cards as well).
Argent Adept: I've had a bit of trouble getting his combos out, but he almost always has something at least marginally helpful to do, and usually better.
Unity: OP. She's easy to use, fun to play, and has hilarious cards, but she almost makes the game too easy to be any fun. ALMOST.
Ambuscade: Surprisingly tough. The whole bit with him shuffling his deck every turn is kind of annoying. He's fun to play, though. The traps are creative, and his flip mechanic is great. (I was going to do basically the same thing for the villain contest, but obviously had to abort that.)
Akash'Bhuta: Probably the easiest of the new ones. I like how it's tied in with the environment deck, and the large amount of hit points make it seem like a long, RPG-like battle, which is great.
Apostate: I've only played him once, but he's neat. He mirrors Fanatic perfectly. His flip mechanic is a little confusing at first, but easy enough once you figure it out.
Gloomweaver: My favorite, thematically. The pins are a huge nuisance, as well they should be. The cultists turning into zombies is a great touch, and the Power of the Grave card makes them a fearsome sight to behold. Also, I like how he's the only villain with an alternate heroes WIN condition, rather than an automatic heroes lose like Voss or Baron Blade. Granted, taking out the relics is barely easier than taking him out (unless you use Cedistic Dissonant) but the option is nice.
Ennead: The hardest of the new set, easily. Especially on advanced. The way they play like an opposing group of heroes is great, and the way the deck dictates what extra actions they take is extremely creative. It must have been hell making them balanced.
Tomb: Awesome environment. I found it significantly easier than Rook City. The fact that the cards target ALL non-villains rather than just heroes like RC helps. The treasures are pretty neat and helpful -- temporary powers FTW -- but there are only 2, so I felt they were underused.
Plane of Discord: Awesome. I like the creative things they can do (and did do) with an alternate universe with wonky physics. Imbued Vitality is pretty hilarious. WE GOING TO GO FIGHT CRIME?
Overall, I give it 7/7 asses.