Devastator squads have only changed in armament. First, they now have access to grav-cannons with grav-amps. This allows them to compete with Centurions as a source of grav. Devastators are cheaper, but less durable and don't fire as many shots on the move. Second, Devastators may now take an armorium cherub. This is a cheap, one use item that allows a single model to re-roll all of its to hit dice for one shooting phase. For the cost, I think it's worthwhile. There's always a turn where you don't score enough hits on an important target. The cherub model only comes with the new Devastator squad, but I have a couple servo-skulls that I plan to use instead.
Centurion Devastator squads, like their assault counterparts, are 5 points cheaper per model and don't come with a mandatory Veteran Sergeant, so the basic squad is quite a bit cheaper. The upgrade to twin-linked lascannons is also 5 points cheaper, but the upgrade to a grav-cannon is 5 points more expensive, to grav-cannon centurions cost the same as last codex (though the squad will be a bit cheaper without the Veteran Sergeant upgrade).
Thunderfire Cannons got quite the upgrade. They can now be taken in squadrons of up to 3, and the Techmarines got the statline of the old Master of the Forge. This is particularly notable because they are BS 5 and 2 wounds, meaning that they should stand in front of their guns now. In fact, you could make quite a durable squad by alternating techmarines and thunderfires, forcing the enemy to chew through an inoperable thunderfire after killing each Techmarine. Finally, if a squad includes 3 Thunderfires, they benefit from Convergent Targeting, which gives them +1 BS. This raises their BS to 6, meaning that you will very rarely miss by much.
Predators haven't changed on an individual level, but they are also now available as squadrons of up to 3 tanks. As a bonus, a Predator squadron containing 3 tanks gets Killshot, granting it Monster Hunter and Tank Hunter. This is a massive increase in killing power, but the enemy can easily take it away by killing one of the vehicles. Even so, it's a free bonus if you already want to take a lot of tanks. A squad of lascannon armed Predators would actually be a decent super heavy hunting unit, as long as they got to shoot before the super heavy could take them out.
Whirlwinds also haven't changed at all as individuals, but can be taken in squadrons of up to 3, receiving Suppressive Bombardment (granting Pinning and Shred) when there are 3 Whirlwinds in a squad. This is a bigger bonus for Whirlwinds than heavier tanks like Predators. First, they're dirt cheap; the reason they weren't taken before was because they took up a valuable heavy support slot. A full squadron is cheaper than 2 Thunderfire Cannons and will still do a substantial amount of damage. Because they're Barrage, it will be harder for your opponent to quickly kill a tank to remove their bonus rule for a full squadron. I think Whirlwinds will be much more useful if players give them the chance.
Vindicators became 5 points cheaper but are otherwise the same aside from their new squadron rules. In a callback to the old Line Breaker Apocalypse Formation, a squadron of 3 Vindicators gains Linebreaker Bombardment, allowing them to combine their shots into one Apocalyptic Blast with Ignores Cover. Obviously, anything hit by this is going to feel it. The trick will be keeping all 3 Vindicators alive until they get into position to fire. Even trickier, they all have to be able to fire. Any Crew Shaken results will prevent a Linebreaker Bombardment, much less a Weapon Destroyed. Even so, the remaining Vindicators will still be able to fire normally, so you're not losing anything. A squadron of Vindicators will demand the enemy's attention, and will therefore draw fire from the rest of your army. I suspect they will rarely fire a Linebreaker Bombardment, but it could win you the game right there when they get the chance to do so.
Hunters can be taken in squadrons, but are otherwise the same. When there are 3 Hunters in a squadron, they get Savant Interlocution, allowing them to re-roll the dice for their Savant Lock rolls.
On the surface, Stalkers look the same. However, their Icarus stormcannon array has changed quite a bit. It lost a shot, but it can still engage 2 targets each turn. If it shoots at one target, it's still BS4 and twin-linked. If you choose to fire at 2 targets, it's not twin-linked, but it still fires at BS4 (rather than BS2 in the last codex). This makes it tremendously advantageous to fire at 2 targets (twice as many shots will statistically do more damage than twin-linked). In addition, it also gained Interceptor, so you can shoot down enemy flyers as they arrive, before they can threaten your Stalkers. Once again, if you take them in a squadron of 3, they gain a special rule. In this case, they gain Skystorm, which allows them to Ignore Cover. This will eliminate Jink saves from flyers, making the Stalkers an excellent anti-air unit. A squadron of Stalkers is the best answer Space Marines have to enemy flyers short of taking flyers of our own, and they completely outclass the Hunter.
NB: I've heard a lot of players talking about how a squadron of Stalkers can shoot at 2 targets per Stalker (i.e., 3 Stalkers could shoot 6 targets). This is incorrect. The rules for the Icarus stormcannon array clearly state that a unit armed with one or more can choose one or two targets. This applies to the whole unit, not each individual stormcannon.
The Stormraven, Land Raider, Land Raider Crusader, and Land Raider Redeemer are all exactly the same as in the last codex.