It seems lately that a lot of people have been having problems using commissars...
Commissars have one purpose in my eyes. That purpose is to keep your silly little guardsmen from running straight off the table if something scary is within a 50 foot radius of them. Commissars also ensure that your 50 man guardsman squad doesn't get cut down as they run away.
So, let's take a glance at the commissar rules, shall we?
Basically, leadership 9, 1 wound, 2 base attacks, 35 points. He also gives his squad stubborn. If you fail that stubborn leadership test, he shoots the model with the next highest leadership value, and you can re-roll the test. A lord commissar has the same stuff, but leadership 10, 3 wounds and 3 attacks. However, the lord commissar is also an independent character, where the regular commissar is an attachment to a squad...
Now, that is one of the main reasons to never ever take a lord commissar, they can get picked out of a unit. They are also 70 points base, with no upgrades.
Now, you can only take a commissar in a platoon command squad, or an infantry squad. Never take one in a PCS, because he will end up shooting your platoon commander, and not really doing anything. The key to regular commissars is putting them in a 30-50 man combined infantry squad. Here is an example:
-50 guardsmen, 5 autocannons, attached commissar
This unit comes in at about 335 points for 51 dudes. It will toss out LOTS of shots, and won't completely crumple in the assault. This is the only place to use a regular commissar, and probably a commissar in general.
The only place to use a lord commissar would be with ogryns probably, which we all know sorta' suck.
-Regular Commissars: Use them in giant infantry squads. Anywhere else they are just a waste of points.
-Lord Commissars: Don't even bother. All the enemy has to do is throw a single S6 attack at this guy and he dies. Then your entire squad of whatever he was with runs off the table, costing you 2 kill points.