So, with the "new" codex, we now have 3 H.Q choices to pick from. A Commissar Lord, a Primaris Psyker, and a Company Command Squad.
Here, I will review each of them, and tell you why only one of them is worth it.
Company Command Squad: This is the one, people! The company command squad may not be the most destructive as the Primaris Psyker, and may not generate such a powerful radius of moral as the Commissar Lord, but if you add all the wounds together, this squad has 4 more wounds than the psyker and the lord.
It can also issue orders, basically using the troops as its weapon. Tell a unit to FRF, SRF, and you're turned a unit of guardsmen into a conduit of destructive focused firepower. A Commissar Lord only gets his pistol...
It also comes in cheaper, and more durable than both the other two choices, and much more versitile in its wargear choices.
You can buy 2 flamers, sniper rifles, or grenade launchers, and still be 10 points cheaper than the other H.Q choices, meaning you get more wounds, orders, and additional supporting firepower.
Also remember - there are two unique SC's that count as Company Commanders, so there are more options for variation in additon what I just wrote.
Commissar Lord: Coming in a 20 points more than a command squad and 4 less wounds with only a single shot in the shooting phase, no orders, and the vulnerability of a storm trooper + extra wounds, the commissar lord is far too expensive for what he does.
I will admit, the Commissar Lord is unparalled in the assault phase compared to any other Imperial Guard unit, but that won't get you far when your opponents wound him more easily than he wounds them, or strike before he does, or both! The Commissar Lord really has only one single purpose - stay in the very back of a maxed out conscript mob to keep them from running away.
If the Commissar Lord is used in any way, he will die. He cannot lead smaller units as they will perish, or he will kill the sergeant, reducing the units effectivness by a lot (4 attacks on the charge go a LONG way, guys! ).
Now about his wargear options - he's only one person, and an "officer" for that matter, so no special weapons. he can take a power weapon or power fist, but it will do little good, because either he will die before he can attack, or he will be kept in the back of your army. Don't use it.
What I would take, is carapace armour. This increases his chance of surviving by enough that it might be worth it. Also, a plasma pistol may be useful before a charge, especially with his high ballistic skill.
Lastly, we have melta bombs. If he is leading a conscript mob, this will be the mob's only chance of killing vehicles, but as well all most likely know, the guard strives upon numbers, and a single melta bomb will usually do little good against armour. Best not to take them at all.
The commissar lord is very useful when coupled with a company command squad, I have found. Do not attach him to it, but instead keep him and his conscripts near another unit, and swell with confidence as your units gain Ld10 for Order tests.
Primaris Pysker: At the same cost as Commissar Lord, but with far less durability, this unit is one I will never use again.
No options at all with this one, and equipped only with 2 psychic powers, the Primaris is nigh useless. He can provide fire support, but at this point in time, people will most likely see him as a threat with the ammount of high powered shots he gets, and take him out at range.
He gets no orders, and a poor invulnerable save, and with only decent weapon skill, his force weapon doesn't do much good, especially because his initiative is just like any other guardsman.
His psychic blast might be good at killing hordes for a moment, but he usually goes down quick.
Really, the only useful thing the Primaris Psyker does, is reduce morale of non-fearless enemies. You can then pummel the unit with mortars and snipers, hoping to pin the unit, but pinning does little in 5th edition.
In short, don't take the Primaris Psyker. The Commissar Lord is a maybe, and the Company Command Squad is a definite!