The elites section is a short but powerful section.  This section, like most of the codex shines relative to the rest of your list and is therefor important to consider in your army composition.  Don't count on it to be the hammer of your army in tournament lists, but it definitely has that potential to shine in more casual games.

First up, Deathmarks.  Good god, that fluff entry where the Lord chastises the Cryptek as being dishonorable for using on.  These guys are the potato chip vector that delivers the delicious chip dip that is the Abyssal Tek.  The FAQ that confers their Hunters from Hyperspace rule onto characters joining the unit just makes these guys a crazy character buff unit.  If they could be taking in squads of 1, they would be.  I went over the Abyssal Tek in the last entry, so that aside lets consider the Deathmarks as stand-alone.

Ten Deathmarks come in at near the same cost as your Immortals.  That's a big consideration when most elites in other armies tend to be significantly higher points cost for identical stat lines with the addition of special rules and/or wargear.  For 2 more points per model you get to deep strike (normal or as a reaction), picking a unit to wound on a 2+ and rapid fire sniper rifles.  Not too shabby really.  Ten of these guys can potentially take down big MC's between weight of fire and rending.  Don't forget that they have access to precise shot so they could very well snipe special characters/wargear out.  No other non-character has that ability in the codex.  Taking these as a ten man 190 point scalpel unit is not a bad idea.  And you can still get an empty Night Scythe out of the deal too.

Lychguards- I still have a hard time wanting to play these guys.  They can be in a Scythe so they have a better transport option now than before the model was released, but its just more points into an already expensive unit...though unlike a 'Rhino tax', there is no such thing as a 'Night Scythe tax'.

So what do they have to make 40 points worth it?  Toughness 5 goes a long way.  Power weapons on sergeants raise an eyebrow for the tac marine, but less on the biker, and lychguard fall into that T5 category of only being wounded on a 5+ and have a chance of getting back up even after being killed. Beyond that they have Warscythes.  Wargear that goes at initiative and is AP2 is few and far between now a days and even though you're I2, you're still higher than I1, so you'll swing before the powerfist termies or oblits. And at st7 you'll more than likely wipe them out, especially with 2A base.

I really feel like the sword and shield was a 5th edition option.  Needing that protection was nice then, but now I think you'll be able to ride the T5 a little harder and just depend on the fact that you'll smoke squads full of power fists and don't need to worry so much about that lone power weapon.  Coming across less Thunderhammer/sheild terminators should bring the warscythe version of these guys out of the shadows.  Admittedly though I still think the best way to run them is with Obyron for the great mobility and guaranteed hit and run like ability.  

Praetorians- An interesting thought to add some speed to the codex, these jump troops can really make hardier units shiver like lychguard.  Sadly, their weapons are unwieldy so without that extra protection of a invul save you can really eat a lot of sand before you get to rip in (really just means you'll have to take the powerfists before you get a real chance to kill them before they kill you).  At best, to play them effective you'll need to use the three layers they have in assault; shoot, hammer of wrath, then swing at I1.

I am really torn with this unit and more interested in trying them than I am lychguard.  On one hand lychguard are going to out swing power fists, but on the other, both Praetorians and Lychguard are going to be going last against everything but power fists so does it really matter that you're I2 or I1 in the world of I4?  Not really.  So what do the Praetorians have?  St 5 Ap2 shots before assault (overwatch and/or regular), s5 Ap- I10 hammer of wrath hits, and then combat. So looking at it in that light, the Praetorians are the better choice considering they have way more opportunities to kill before combat really even happens.

I think the choice between Lychguard and Praetorians is a playstyle choice more than anything.  Do you like a broadsword or a rapier?  And play it that way!

C'tans-  Honestly the only thing that was cool was Writhing Worldscape and even that was a gimmick. Now that you get armor saves against dangerous...meh..  I think these guys might have phased out in the new edition.  I really can't justify the points for the effect on the game.

Flayed One Pack-  Even relooking at these guys in the new edition, its hard to find a good purpose for them.  They can swarm, thats for sure at 13 points, and if you bum-rush 20 of them into something, 3A base is going to really cause some problems even if its only I2...but I can come up with much better places to spend your points.  Perhaps if they could be scoring somehow, I'd be more likely...but like the C'tan....just not seeing it.  I think both these units suffered the 'it was cool last codex so now it has to suck to sell models' symptom of new codexes.

Triarch Stalker-  Here an interesting model.  In 5th, I was never caught dead without one. It was the center of my army.  Now though, since a large amount of my army is twin liked Tesla Cannons, the twin linking ability just isn't that interesting.  Combine that with the fact that I can glance vehicles to death so easily, the melta shot isn't nearly as important.  This guy used to serve as my land raider getter, but now the command barges do just fine at that since the rest of the army took up the job of rhino killin' duty which was the command barges old job.  Its like the CEO of the tank killin' company retired and everyone under moved up the corporate ladder.

The Heat ray is out classed by the command barge.  The flame template is retired by the abyssal tek.  The heavy guass isn't needed when Rhinos can be glanced to death and the command barge flips tanks like as short order cook flips burgers. the horde killing ability of the particle shredder isn't needed when you have the weight of fire the army can bring.  At the end of the day, he just lost his job...which was twin linking massive foot slogging numbers of immortals/warriors and in my current build they just aren't there.

Really though, this was a boon to the army because it took away the key stone.  I used to really struggle when people learned 'kill the stalker ASAP' and my army lost a huge support unit.  Now I just look at the Stalker and think...that's almost a Night Scythe & Immortal squad....

I think the elites section has some fun units in it for casual play, but in the tournament scene its all about deathmarks.  I think Preaetorians and Lychguard have potential for experimentation and mastery, but for my style its deathmarks all the way, bottom line.  Perhaps a Stalker or two in a warrior heavy army but even then your problem wont be from the stalker; I bet he'll shine.  Its the fact that you have a warrior heavy army that will lose you games.