Bloodhall Priest: Solid. You have to do a lot to start pinging your opponent for two, yet that can end games. I see it as a fun junk rare at some point, though. The lower Madness cost is a good feature.
Mercurial Geists: Weak, yet a nice addition to the instants and sorceries deck.
Campaign of Vengeance: Love the effect, but not for five mana.
Grim Flayer: That's one powerful bear. Trample on it is only useful once you are able to trigger Delirium.
Gisa and Geralf: One of my favorite cards of the set. It can be ANY zombie card in my graveyard. I love that!
Heron's Grace Champion: Flash is nice. The +1/+1 counters are not permanent. Reads like a junk rare, yet still a very good battle trick.
Mournwillow: Nice, I really like that it has haste.
Tamiyo, Field Researcher: I can't wait for it to go down in price, which I am not sure is inevitable, but I wish it so. Three different mana to cast means you need a ton of mana fixing. The abilities are OK, but not amazing. It reads like a fun card, not a strong card. The ultimate is great, though.
Spell Queller: Flash and Flying, what more do you want? You want to exile your own spell? I need to think a little more about this one.
Ride Down: Good against Menace, otherwise, Trample will be useless.
Geier Reach Sanitarium: Can only use one at a time. You need to be careful with it since you could be helping your opponent a lot.
Nephalia Academy: Interesting Madness hoser. I need to see it in play. If your opponent is not asking you to discard, it does nothing. I see it only in my sideboard.
The Meld cards remind me of Pokemon HeartGold SoulSilver. Those double pokemon cards were not that popular, and I expect that to be the case for these. They will make great collector items, though. A flaw Magic players will learn from Pokemon players is that you want to pull both halves but very often you will only pull one... remember this.
Gisela, the Broken Blade: Take out the Meld clause, and you have a so-so Mythic.
Bruna, the Fading Light: A junk rare only worth playing to get Meld to work. It can also go in an EDH deck by itself.
Brisela, Voice of Nightmares: Nice name. Of course, if you pull off the meld trick, this thing is awesome. I am glad there is only three of these. And I am looking forward for these to be the last three two. These gimmicks are worthless complications.
Graf Rats: Limited.
Midnight Scavengers: Limited.
Chittering Host: Limited. You have to pull a lot of cards to pull this off in Limited, though.
Hanweir Battlements: It takes five mana to get the Meld train off the station. I hope I don't pull one of these as one of my rares in a pack.
Hanweir Garrison: It's an interesting rare, never mind the Meld thing.
Hanweir, the Writhing Township: Very nice if you can pull it off. It's doable in battlecruiser Standard.