Let's do this in alphabetical order.
Aurelia, the Warleader
I am impressed that both legendary Boros angels (Aurelia and Razia) are flying, vigilance, haste. I love hasty generals, but the vigilance is far less relevant for Aurelia than for Razia. I would have rather had higher power and toughness over the vigilance. A 3/4 for six mana is not the best starting point.
Realistically, though, as long as she doesn't die to Spirit tokens or Lightning Bolt, her power and toughness are not what you're paying for. You want to attack again, and well that you should.
Red/White has had similar effects on enchantments and sorceries and creatures, and Aurelia delivers a worthy successor to those effects. These are also very aggressive colors, and she is an amazing general for leading the charge.
You're going to see a lot of Aurelia going forward. She brings you into not only a 'multiple attacks' deck, but she fits a 'voltron' theme well. That sort of deck seeks to build one huge creature and kill other players with it, using Auras or Equipment. It doesn't matter if Aurelia is attacking alone or as part of a horde, you'll get extra attacks. Win!
One super-neat thing about Aurelia's effect is that she says 'untap all creatures you control' instead of 'untap all creatures that attacked this turn.' This means that you can use assorted tap-ability creatures, then get extra uses out of those creatures.
Her casting cost is about right for the power of her ability. Six is almost cheap, eight is unpleasant, and ten is rather annoying. So make sure you take someone out with her attacks.
Final Verdict: Great as the Commander or in the 99.
I have to admit, I'm severely disappointed in this card. The first incarnation of Borborygmos was difficult to cast and use--and I tried in two different decks--but the new one, I'm sad to say, is worse. Switching the power and toughness doesn't make up for the extra mana, rising him to eight mana.
Let's talk about his two abilities. For hitting a player, you don't get to draw three cards, you get the lands of the top three cards. If you have gotten to eight mana, you probably don't need more land. Until your general gets killed, and you need ten mana.
His second ability, which lets you discard a land to Lightning Bolt something, is interesting. I'm pretty sure that unless you're hitting Life from the Loam over and over again (and that would be awesome if you did!) you're not going to get too much out of it.
I feel like the first ability should have been a lot more broken. Without a lot of help, or players who don't have enough removal, this guy won't hit very often. The reward should have been to tutor for two basics, or maybe tutor for any nonbasic land.
I know that some people will build a deck with 99 lands and this guy, but that's still going to be a very, very slow deck. Borborygmos Enraged has no way to protect himself, some immediate effect, and is going to be difficult to build around.
Final Verdict: Marginal as a Commander, and probably in the 99 too. Unfortunately.
Lazav, Dimir Mastermind
I love this card. This is the general that will often go into the mill decks, over Oona, Queen of the Fae. Lazav is cheap and self-protecting, which is one heck of a combination.
The mana cost is a little tough to lay down on turn four, but the payoff is going to be pretty tremendous. His ability costs nothing, and triggers off of cards milled, discarded, destroyed, anything!
A warning, though: You won't get 'enters the battlefield' triggers when you change him into something, so be cautious. EDH players love creatures with spell abilities, and those are usually 'comes into play' effects. I know that I always want super-awesome-effects on a creature!
Nonetheless, Lazav is the real deal. Sure, he's vulnerable to board sweepers, but there's only four legendary creatures who are indestructible anyway. (Ulamog, Avacyn, Sapling, and Konda, if you want to know) I'd be inclined to play a couple of black regeneration tricks with Lazav, so in case of Supreme Verdict, you can have him become anything that was on the board.
What I really love about Lazav is how you can have a second plan of attack, or go after two people at once. With Lazav out, you mill Player A for some cards, and flip over Avacyn, or It That Betrays, or whatever awesome huge card they have in their deck. Then you attack Player B for much greater general damage! Just amazing.
Final Verdict: Outstanding as a Commander, and great in the 99 too.
Obzedat, Ghost Council
Strap in, folks, because Orzhov just got aggressive.
On first impression, Obzedat is merely very efficient. Five mana for a 5/5 with a small drain life effect seems good enough, but the devil--or Spirit, in this case--is in the details.
The first Ghost Council of Orzhova was very similar, with an efficient power/toughness to mana cost ratio and a repeatable slow life drain effect. Obzedat takes that further.
Unless your opponents are big Rout players, Obzedat will never die to a board wipe. Never.
Obzedat might as well say 'protection from sorceries' because unless you allow it, he will never be in play to get killed by one. What's more, this protection doesn't cost anything. Not mana, not life, not creatures to sacrifice, nothing. All you have to do is say that you want to use his ability at end of turn.
I have to imagine that since Extort says 'each opponent' and Obzedat says 'target opponent', the developers tried him out as a mega-Extort and found it was far too good. I can see the 6 points a turn being a bit too good, but I would have rather had 'each opponent loses 1' over 'target opponent loses 2' just so it would fit Extort better. That's probably just nitpicky. I tweeted Aaron Forsythe and he said it wasn't too interesting a story.
I think you will see a lot of Obzedat in the weeks ahead. Dodging all sorcery-speed interactions seems very good to me, and the life gain/loss is not too bad either.
Final Verdict: Very good as a Commander and in the 99.
Prime Speaker Zegana
If you ask the average EDH player what things you need to do well in games, you'll probably get an answer that involves enough mana, big creatures, and a full hand of cards. Zegana gets you two of those with only a little help.
Zegana's usefulness is heavily dependent on what else you have in play when you want to play her. As a six mana 1/1 that draws you a card, she's terrible. But with even a 2/2 in play, she comes in as a 3/3 and draws you three cards. That's more manageable.
But that's her low end. For her high end, things are ridiculously high. I've enjoyed Momentous Fall and Soul's Majesty as cards, but Zegana not only draws you a stack of cards for having a fattie out--she then BECOMES a fattie herself! Even a little bigger than the one you had out before!
In blue/green, we have several choices about creatures who have power and toughness equal to your hand size. My favorite with Zegana, and likely a popular combination, will be with Psychosis Crawler. I draw, you lose.
Keep in mind that her first ability is not triggered--you need to kill big creatures in response to her being cast, and Aether Flash will not kill her. If you can do something to lower her power in response to her second ability (which IS triggered) then you should do so for real hilarity.
Her limitation, of needing another creature in play to be truly effective, is going to sometimes cause an awkward board for you, where you have lots of mana but no creatures, probably due to a board wipe.
Your task, then, is to jam mainlands into a deck with Zegana. Treetop Village, Faerie Conclave, Mutavault...the list goes on.
Final Verdict: Awesome as a Commander or in the 99.
In a week or so, I'll give you the card-by-card rundown for all the Gatecrash cards. My fingers already hurt. See you then!