There are two advantages I can think of for being able to don/doff armor as a an action:

1. Avoiding Heat Metal.
2. You can’t sleep in your armor, so if you’re attacked while resting, you can put it on, but the rest of the party will have to manage without it.

Both are situational, but the second one could happen often if you have a DM who loves to interrupt long rests—and mine definitely does.

The old level 9 improved armor feature let your arcane armor count as 4 different pieces (boots, helm, chest plate (armor) and weapon) and each piece could be infused with its own infusion. Along with the 2 extra items you could infuse.
This old feature would let your arcane armor get stronger (eventually +2 armor, +2 weapon, helm of (take your pick) and boots of (take your pick) all on your arcane armor)

Less than 20 seconds in and slandering the alchemist. What a power move.

This subclass, I think, suffers the worst from the 'but any class can just craft or buy better items than anything the artificer can replicate' problem.

The one nifty thing is that since your armor is both your focus and your weapon, and they removed the verbiage about 'only counting as a weapon when your hands are free' part, you could just equip two shields. I know you wouldnt get the AC bonus from both of them, but like a repulsion shield and a spellguard shield would both be good to have at the same time.

3:13 How the armorer does not get shield is still utterly baffling to me.

I love the Armorer artificer and their ability to be a tank or scout. The dreadnaught model is kind of strange with slightly better damage, although still low, and small amount of battlefield control with push and pull

I know the standard is to look to Iron Man for inspiration for the Armorer, but I really can't stop thinking about Ruber's minions from Quest for Camelot when I see the Dreadnaught with spike ball arms, and that brings me joy.

This I'm particularly interested in because it's one of 2 subclasses I'd multiclass from the artificer with the warlock. I might tinker with the cartographer artificer though

One thing the Armorer really needed was the same new feature that the artillerist got, letting them switch modes outside of short/long rests.  It would be fine if it takes a bonus action or even a full action, but being able to switch between modes would add a lot of flexibility to what you can do.  For example using stealth mode to scout the enemies or to close the distance with faster movement speed, then switch to demolisher or tank to kick off the fight.

One of the players at my table is playing a 5e our DM has always said he can upgrade to the 5.5e when this comes out, so he's very excited right now! Between his incoming Dreadnaught armour and my Goliath World Tree Barbarian, we'll have a couple of big beefy boys in the party!

The Guardian’s THP used to have limited uses. The fact that it’s unlimited actually makes the guardian armorer very durable once they’re bloodied. It also makes them a great candidate for the feats from Heroes of Faerun that give you benefits while you’re bloodied.

1:17 you can also craft common armor of gleaming from level 1, so I think this is meant to be like a very long time and money sink to make early rare armor (dwarven plate or dragon scale mail come to mind) as kind of a "final armor" for you while you use prototypes

You can't sleep in heavy armor. So donning/doffing it with an utilize action in the middle of night when your tent is being ripped apart by a hungry owlbear seems like a huge benefit to me.

8:10 You do get Repulsion Shield and Sentinel Shield at Level 6

Im sure its not crazy useful, but thief rogues can use the utilize action as a bonus action, meaning with a 3 level dip, you could don your armor and cast a leveled spell or take the attack action in the first turn of combat, if you were waken up mid-long rest, or if you can store your armor on your person for social infiltration gone sour

I think the infiltrator's 15 level upgrade is meant to be more of an weirdly upgraded bonus action dash? It feels very much like Dash in that you really want to use it if you need to get somewhere as soon as possible and with less issues since you can fly as oppose to I can just fly for a few minutes like the usual fly abilities tend to be. It's definitely an interesting way of doing it.

I think a multiclass rogue (arcane trickster) / artificer (armorer) would be very cool and dangerous, using the infiltrator model. The lightning emitter qualifies for sneak attacks. And permanent stealth advantage is very sweet for a rogue.

the level 2 replicated magic items allow you to make any common magic item other than potions or scrolls. there are 4 types of armor that qualify:
armor of gleaming
cast off armor
shield of expression
and smoldering armor
in addition to:
shield +1
armor +1
sentinal shield
repulsion shield
and arguably
helm of awarness

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Arguably, every spell on the armorer's spell list ranges from good to best. Absolutely bonkers.