Are Commissars worth it?
1. Not IC, so cannot be specifically targetted in hth. Course, IG officers now have the same status, since they're retinue is a retinue and they count as upgrade characters until the squad is gone. But yeah, they can be a hidden ST 6 powerfist/ST 3 power weapon.
2. Assuming he's in a Command squad, +1 Leadership boost for units within 12" of the officer. Note that the +1 boosted LD does NOT go down the vox (see the IG FAQ).
1. Not IC, but they are a unique model, so you will have to allocate wounds to them if the unit receives enough wounds. And those wounds cannot roll over onto the squad.
2. Cost. A well-equipped Commissar runs as much as a squad with a heavy weapon. I'd rather have more guns.
3. Unit allocation. Assuming advisor Commissars, you are limited on where the Comm can go. 1st one MUST go to your HQ Command Platoon Command squad. Then you go to Infantry Platoon Command squads. Finally, depending on your army composition, you can get them into those other units, like the Infantry squads or Armored Fist squads. SO is it worth it to buy the 1, 2, 3, or even 4 Commissars you'll need to get one into that non-Command squad? Goes back to cost, that's how many guys with guns I can't take? Also note that they cannot be attached to certain units, like Ogryn, etc. Only those units listed in teh Advisors grey box can have them, unless you use up a Doctrine slot and spend even more points on them to make them Independent.
4. Summary Execution. Sorry, but this is nothing but a drawback now. Let's take that example someone mentioned, the Conscripts with an Indy Comm (who can be picked out in combat and as an IC, must pile in first and try to get into btb with the enemy). They get assaulted and lose, fail their -7 Leadership test, the Commissar does a summary execution and pops Joe Trooper. The unit will now take 7 more wounds due to No Retreat, and stick around to fight another round. Second round of combat, now under the Commissar's leadership, they lose again, we'll say only by 5, and now they fail their Morale again. Ooops, already used the summary execution (after all, he's leading the squad now, and he's not gonna shoot himself), can't do it again, so it's run away time, leaving the uber-assault unit free to move and assault again during his turn. For those who keep saying "never run away", wrong, it's won't run away for one player turn, after that, they can and will run. I'd much rather run away the turn he assaults and shoot them down after my unit flees.
So bottom line.
No, Commissars are not currently worth it, regardless of how cool they look. But I keep hoping, still have 15-20 of them, so maybe in the next codex............................