Dont use them, but buy them and wait for the 5th ed codex. The way I see it they are a burden to have right now with the whole if you auto pass your ****ed up the bunghole rule. If they dont want every guard player to hate them GW will make comisars better or cheeper. Right now the best way to use them is in a squad of him and Hardened Vets, the vets have pistol/CCweapon and the sarge and Comisar both have power fist/Weapon. It sucks up points but it can be so fun. The trick is to make sure they are the ones charging and have a higher init if posible, 30 attacks! 4 Power weapons! OMFG! my Orks just got raped!, that is assuming we are not talking a full 30 mob, more like 15. Without Nobs.(EQUIVALENT) And assuming the non power cadys are at least getting a 50% hit-wound/Attack ratio(Generous I know). Assuming that works oout you have the potential of taking out a squad or 2 without casualties.
That said, its a crazy Idea and should ony be attempted if you are verry lucky, verry good at pulling things off, or are plaing against a dumb player.
My final verdict: Not worth it at the present time.
Wow I rambled on that one!
"A true king is never alone, his will is equal to the will of all of his followers."
-Alexander the Great, kind of