Right off the bat, we have three legends that are the same characters that we saw the last time Magic went to Ravnica. In case you're unclear, you can play both legends on the table, as long as they have different names. Planeswalkers are different--only one "Planeswalker - Jace" may be in play at the same time, but the legend rule refers to the card's name, which has to be exactly the same for them both to be put in the graveyard. It's perfectly fine to have both Isperia, Supreme Judge and Isperia the Inscurable on the table at the same time.
In (almost) alphabetical order...
This little lady is many things. For one, she's more powerful than her first version, Isperia the Inscrutable. Isperia 1 needed to hit a player, and usually twice, before a creature with flying could be tutored up. In my experience, her 3 power was the big drawback--it wasn't too hard to block.
The new version is 6 power for 6 mana, and flying. That alone is a good deal. The kicker, though, is that you attack without fear, because people won't want to attack you back and have you draw cards.
W/U decks can easily fall into a 'control' role, where they can wipe the board and counter problem spells, and playing this legend means that until she's gone, the other players will likely beat on each other. Be warned that some players will see her as a challenge, and token decks will be happy to let you draw 50 cards at once. My verdict is to play this in most W/U decks.
Jarad, Golgari Lich Lord
This is an intriguing general in so many ways. He heads up two popular and powerful tribes, he obviously enjoys decks with lots of creatures, and he also packs a sacrifice effect that can do a lot of damage very quickly. I can see him being a worthy second-in-command to Savra, Queen of the Golgari sacrifice-based decks. I think Savra would be the general, and Jarad the first tutor target, because of his built-in recursion.
He's undeniably powerful, and just gets better as the game goes on. Important to note that he can't sacrifice himself, so you can't slap him down then drain the table for some huge amount.
I think Jarad will get a lot of attention as a general, because of his abilities and because he came in a duel deck as a nice foil.
Our second go-round with the Firemind is another doozy. For EDH purposes, the original is renowned for combo potential, and hated on for that reason. This version is a bit more difficult to abuse. It's possible to combine draw and damage effects (Quicksilver Dagger, Thornbite Staff, Ophidian Eye, etc.) but this is a less powerful version of Niv, make no mistake.
I'm not sure if you want to give him double strike, or focus on his pinging ability. I'm glad they put both colors of mana into his activation cost, as there's ways to lower a colorless cost. I do wish that his ability said "Draw that many cards" in order to encourage attacking, but I can see that being overwhelming.
I think he'll see some play, but I suspect that people will gravitate more to his original, overpowered version.
G/W decks have a lot of choices for generals. There's Captain Sisay for the legend decks, Chorus of the Conclave for +1/+1 counters, Gaddock Teeg for messing with everyone else, the new enchantment king, Krond the Dawn-Clad, Saffi for graveyard combo decks, Sigarda for hexproof decks, Tolsimir Wolfsblood for "Here, Voja!" decks, and Rhys the Redeemed, grand poobah of token decks.
This dryad is a worthy addition, and might be better than Rhys for your token decks. She's not very good for non-token decks, because her ability needs the tokens to be worthwhile. In a big-mana deck, with Gelatinous Genesis, Wurmcalling, and the new Ooze generator, Trostani threatens to overwhelm the board quickly and gain a lot of life. This is another legend that I think you'll see a lot of, though it's possible that people will go back to Rhys's mega-populate ability quickly enough.
Rakdos, Lord of Riots
Wow. Just wow. This is a ridiculously powerful creature, undercosted no matter the drawback. A 6/6 flying trampling nightmare. I think this is powerful enough to see Standard play, but the EDH applications are where this card gets really silly (and might be too good!)
First of all, there's the Kaalia decks. Kaalia attacks, putting Rakdos in as an attacker for free, and dealing 8 damage. Now your angels, dragons, and demons play for their colored mana costs! Avacyn for WWW anyone?
If this guy is your general, then the sky is the limit. There's a lot of black and red cards that will make each player take damage/lose life, and the combo potential is very, VERY high. My favorite would be Acidic Soil, play Rakdos, then play Ulamog or Blightsteel for free!
The original Rakdos, the Defiler is a winner in Kaalia decks too, getting around his drawback and leveling someone's board. I don't think I've seen a deck where the Defiler is the general. Most B/R decks I've seen have been Lyzolda or Kaervek...but the Lord of Riots is going to be popular.