At this point, I think everyone's at least aware of what's in the Indomitus boxed set. I've had a chance to muck around with the marines a bit, and here's where I think they'll fit in. Note this is in advance of a new codex, and we also have plenty of rumors and teasers of re-worked weapons profiles anyways.
Primaris Captain
Hey, look at that - the marines got a captain in their side. There are a handful of other power-armor captains out there. You get a heavy bolt pistol, a two-damage S5 power sword, and a relic shield that ends up granting you a 2+/4++ with a 4+ save vs. mortal wounds.
This guy's a niche piece, I feel - you can get more melee punch and speed off the good ol' hammer/shield/jump pack captain. The powerfist/plasma pistol captain has better strength on weapons. The vanilla sheet also gets you better ranged guns. But, if you're going to wade in and thrash, the relic shield makes this guy worth considering.
Impression - consider the captain if you're skipping a 'smash' captain and going try to crush face in melee.
Primaris Lieutenant
So, full confession - we all love those Primaris Lieutenant memes, but this guy actually has a pretty solid loadout. This guy has a decent melee weapon (S5 2-damage power sword), an invulnerable save via storm shield, AND a pretty solid pistol. I mean, come on - it's two shots at S5, two damage, and potential mortal wounds.
If you're going to take a primaris lieutenant (and unless you're going for a jump pack, might as well go Primaries) give this guy a look.
Impression - I honestly think I'm gonna end up using this guy a fair amount. He's got a decent gun, and unlike all his other lieutenant buddies, he's got an invulnerable save.
Primaris Chaplain
Um, it's a chaplain, fellas. We already have this. It's a neat new model, but doesn't bring anything new.
Judiciar
Now, THIS is something new. Sure, he's got melee punch. Here's what we care about, though - "You, you there, within 6 inches - you go last." If you are on foot (Maybe if you're Iron Hands and still trying for the whole mob of intercessors), the enemy basically HAS to hit you with as many elements as they can at the same time.
Why? Well, one of them goes last. Another one swings, and then you can pop your 2CP interrupt stratagem. If you're in a foot mob, or delivering a large group of guys via transports, then bring this guy.
Impression - it's build-specific, but definitely worth bringing if you think your mob of troopers is going to get charged.
Bladeguard Ancient
This is basically a riff on the existing Ancient, except that it provides Bladeguard-specific buffs. Frankly, the "act again on a 4+" isn't super-reliable, and the utility depends a) on stuff dying, and b) stuff dying when its retaliation is meaningful.
For example - bladeguard dying and shooting a pistol is kinda 'meh.' Hellblasters gettng punched to death and swinging a couple times in melee is 'meh.'
Impression - The codex may bring more, but consider this guy only if you're already bringing a couple bladeguard squads.
Bladeguard Veterans
First, first impression - um, what's up with 3W? You're not Gravis guys. That aside, at first glance these guys are in a wonky spot. They're a small unit. However, they're kind of sick in melee - everyone's packing a storm shield (even if it is the weird new-ish one) and a master-crafted power sword. If you're bringing these, consider bringing a pair.
Realistically, these aren't for every army - if you try to walk them across the board, expect someone to thank you, and shoot them off the board if they can. Build around delivering a couple squads into the enemy, though, and maybe you'll enjoy 'em.
Impression - Some chapters will love them, others won't necessarily care - but don't bolt them onto any build and assume they'll do great.
Assault Intercessors
Well, it's about time we got stabby intercessors. I have to assume that the codex will let Sarge take melee options, especially since regular Intercessors already have them. These guys will be fine for knocking enemies off existing objectives with fistfuls of dice (outside of anything else, you're cranking out 4 dice a dude on the charge at AP-1) if you can get there. Note that if a volume of attacks is what you're after, you can always look at the auto bolt rifle on regular intercessors. You lose the chainsword and fancy pistol, but go up to an assault 3 gun with 24" range. You also get a stratagem to auto-hit all your shots at 12" range. You don't always, y'know, WANT to be punching someone to death - and you still aren't a slouch in melee.
And that 'if' means that these may not be for everyone. It's the same idea as Bladeguard vets - you're specialists. Build around it, or don't take them. And ask if you're ready to give up a rifle before going into melee.
Impression - I think a lot of armies will keep to existing intercessors, unless you've got great melee rules (cough cough, white scars, cough cough). Otherwise, if you want volume of attacks, the assault bolt rifle's an option.
Outriders
I'll admit I'm partial to these guys. They're fairly beefy in terms of toughness and wounds (T5, four wounds a guy), but note that bikes interact with cover differently than regular infantry. You want to break LOS if possible on the approach.
And believe me, you want to make that approach. If you want a chaff-clearer, it's these guys. If you make the charge with a full unit, you're throwing out a hilarious 19 attacks with S4 AP-1 chainswords that hit on 3+. It sounds like a lot, but it's reasonable to assume you're operating outside of support. You'll be doing all this after firing a half-dozen bolt-rifles' worth of shots into the target too. All told, that's a dozen AP-1 bolter shots onto your target first.
In spite of the volume of attacks, remember - these guys are probably going to run screaming well out of range of any re-roll auras you have.
Based on the volume of attacks these guys get, I don't think we'll see Sarge getting melee options. 5 Thunder Hammer swings on the charge on turn one seems...unlikely.
These guys will be useful for putting pressure on objectives early, or going for stuff like 'Engage on All Fronts.'
Admittedly, some armies will get more mileage out of these guys, but I think everyone can get something out of them.
Impression - I'm sold. I won't run these all the time, but I can see running a couple units of these just to mess with someone else's plans.
Eradicators
Let's be real - these are the guys everyone focused on when Indomitus dropped. Honestly, it's understandable - the mere presence of these guys is meta-shifting. You have to assume that a squad or two of these guys is going to walk off the side of the board and double-tap something important.
A 24" range is just about impossible to screen against, unless you're using terrain or stratagems. You can probably keep them out of melta range, though. Also god help you if you managed to leave a character exposed, someone's going to have to get a bucket and a mop.
Obviously their natural prey is enemy armor, monstrous creatures, and really, really beefy infantry. You'll rarely ever skip out on Total Obliteration.
Why? Unless you're starting these guys on the board (and hiding them well, just in case you don't go first) these guys may not be in re-roll range.
Expect one good round of shooting out of these guys. They're reasonably durable at T5 with a 3+ save, so if the enemy can't shoot them to death, they may just assault them. And Eradicators have....a pistol...for close combat. So, y'know, try not to get into a fistfight. And expect the enemy to try to charge them to tie them up if not kill them out-right.
Army-wise, any Marine army would be happy to run a couple units. Note that anything that makes them more accurate is great - Salamanders come to mind, for reasons mentioned above.
Impression - Most marine players will try these guys a few times. Not everyone will ultimately keep them. Others will eventually adapt to them, one way or the other.
Overall Impressions
Eradicators drew all the attention first - and it's not undeserved. That being said, I'm partial to the Outriders myself. However, don't sleep on the Lieutenant - it's a solid loadout. Finally, most of the rest of the box has at least a situational use.