So on the whole, you're getting about half a card. Seems pretty darn good, but not broken.
This is my early pick for best blue common. Between your own ETB effects, opposing werewolf transforms for a zillion mana, Emerge creatures, and the occasional permanent buff (creatures that get counters from when other stuff dies or is sacrificed, or might I suggest the card right above this one), there's going to be quite a bit of value to be had, I think, when you get an extra card to boot. But even in a bad case, bouncing any creature with CMC of 4 or more is a tempo-positive cantrip, 3 is tempo-even, and even bouncing an opposing 2-drop is a 1-mana-down cantrip, which isn't the worst thing ever.
Casting one of this is obviously bad. Casting the second is obviously good. Does getting to cast the second make up for having to cast the first? I think so, but it isn't fantastic - 4 mana for +1 card is below rate, but you do get to split it up, and I expect this to be a bit of a slow format. Compare to Courrier's Capsule. It's worth pointing out that Mill/Looting and to some extent discard can help you out here. But I still don't think I would run these almost ever with less than 3 in my deck. You won't get 3 very often (on average, only 2 will be opened in a draft), so I wouldn't pick it high, but you can spend a mid-pack pick on one if you tihnk another will wheel. If you ever get to cast 3 in a game, you've obviously done it, with a split-cost Opportunity.
It's worth noting that this is really the best chance you have in this set of reliably getting an enchantment in your graveyard (at common, since SOI commons are now rarer than EMN uncommons), which is fairly important for Delirium. It also kills 80% of the format (or more, if you include tokens). Not only is this going to be the best Black Common, it won't be all that close. I'm fairly confident there's a bigger gap here than in any other color.
I think I've learned my lesson from Angelic Purge and Sinister Concoction: This card will be good. Now, it is worth noting that between costing 5, there being less madness overall and in black in particular, and actually wanting your lands in play because of additional mana sinks, you're going to get to mitigate that discard cost much much less than you did with the Concoction. This is also worse for Delirium. On the flipside, though, the "sacrifice a creature" part is actually less painful now - we have a lot more fodder now, probably largely because they needed to make Emerge work. You can't load yourself up on too many sac effects without enough fodder, but I think you can make this work.
It's also worth noting that you need them to have two creatures for this to be not bad - there's no 'up to' clause. Still, that's not uncommon, and 3-for-2ing yourself when the 2 are their two best creatures, has got to be worth it, in general. So I expect this to be one of the better black uncommons, while still far far below the top dog (Murder).
I think a good comparison here is Fireshrieker. That card was pretty good, but not absolutely amazing. You could pay its cost in installments, but once you were going to the second creature, it cost more overall. It was colorless, though, and importantly, it worked on defense. So it's hard for me to think this will be a good card, but with the number of boardstalls that seem likely, it will probably be at least playable.
As I noted in my mathy review, this kills about 20% of the creatures in the format. My guess is that that's enough to make it begrudgingly main-deck-able, or more likely, a card I'd like to have in my sideboard. Worth noting that I think it would be pretty playable as an instant (where it can block and kill an x/3, with some options for upside), or as a 1 mana card. Also, this does have some applications in "finish a creature off, get a devil", but.... a devil for 3 mana is too bad to help you on such a minor trade-up, I think.
This card is very good. 3 mana shock is a bit below rate, but would get played reasonably often anyway. 4 mana to deal 3 to a single creature is in a similar reasonable-not-good territory. The option between the two is already getting to be a fine inclusion. This also has the board-sweep option, which is a nice little plus. Four mana is going to be very common - killing one thing, and trading up with one or more others (and picking off X/1s). People are going to learn to expect this and play around it - that'll be a key skill in the format. Problem is, it's going to be pretty touch to play around most of the time.
The big thing I want to point out, though, is that the fully escalated version is actually an overrun variant. 5 mana, probably kill a thing, all your stuff gets trample, and hits in for 1 more because you pinged their stuff. Not as big pump for sure, and scales off their blockers rather than your attackers. But one cheaper, instant speed, and probably most important, with lots of flexibility.
There's enough sacrifice that I expect this to make the cut... probably a bit more than half the time. Friendly reminder that clues count.
The nice thing about this is that you can just "raw dog" it on turn 2, and you'll almost never miss. With very few Vessels of Nascency anymore, this is going to be one of the better ways to get to Delirium, which is quite a bit harder now. The more important this is for you, the better this card is.
I think this card might actually be reasonable. Transform costs are a lot more than this, and flipping is very much like a combat trick much of the time. The thing I really want to note, though, is that this is the only way to get the new eldrazi werewolves back to the front side.... though you'd almost never want to. I guess someone somewhere will at some point use this to get around a Humble the Brute or something, and I will be happy then.
People seem to think this card will be very good. I look at the font side and see... well, 3 mana for an 0/3 and a 3/2 is fine, but not worth sacrificing a permanent, unless you're getting big value from that. We see lots of fodder in this set, but I'm still rather skeptical. It's certainly not a turn 3 play. But I think that, in general, to be good enough, you have to have a reasonable chance of turning on Delirium. I am thinking that's going to be a good bit harder now than it was before, which makes me think that this card is middling. If you can get it fairly reliably, then the card becomes quite good.
I think this is getting slept on significantly. 3 mana 2/3 is only slightly under where we want to be. Tap to mill for 2 is fairly powerful in the format. If you can ever flip it, the back half is really game-winning if you aren't massively behind - and even if you are, paying 3 mana for a 4/6 is more or less better than you can hope for. Hard to imagine the card can be too great, since it will be a bit tricky getting enough colorless creatures to make this reliable (remember, you only count front faces). That brings the average number of colorless creatures per draft to 11.6. I expect you can get a disproportionate number of those, so let's say 3 on average. That means you'll usually draw or mill one within a couple turns of this being out, which makes it such that you're reasonably likely to flip it by the time you'd be flipping a wolf (assuming it isn't too hard to trade a critter off) - but with no additional mana cost.
I'm actually going to go ahead and predict this will be the best green common, over Prey Upon. I wouldn't be surprised to be wrong on that, but they're at least in the same ballpark. Mana dorks are typically good anyway, and this format has tons of mana sink. This has the extra bonus of being quite relevant in the late game, too. It's just a win/win/win.
People seem to think this card won't be good. I don't think it will be amazing - 3 is kind of a lot to equip - but +2/+2 is a big bonus, and if it's slow and board-stall-y, as I expect, this one will be above average.
People are really scared of the rider here. Obviously, that's not a good thing for you. But you can plan for it, and this creature is just sooo big. Artifact creature also helps delirium. You have to be wary of enchantment-based removal here, which probably makes this a tiny better if you have sac effects to get out from under it, but I think that just by itself, it's already something I'm never cutting form my deck, and usually pretty happy to have.
And the last card is one I think is pretty overrated. Yes, gravedigger is usually pretty good. Adding a mana, power, and toughness.... you can debate if that's good or not - I think it's a weakening. But being limited to CMC 3 or less is a real cost, and one I think is being underestimated. There are a couple things it stops you from doing; first of all, you can't loop this thing. So your endless loop of value has been cut off. Possibly more importantly, you can't get actually good creatures back with this very often. Finally, you're going to have 0 legal targets for this, which is a molten disaster, way more often than with normal Gravedigger. So I expect this to be a mid-late pick (6th-10th ish), and while totally playable, also definitely cuttable.