There is a huge gap between partisans and irregulars. First of all different doctrines, secondly their manpower, their equipment and lastly where they can spawn.
Since you said you mean irregulars we should use their name to not get confused
Irregulars cost about 180mp and spawn from the ordinary spawn points, where everything else like conscripts, support teams or tanks do as well. They are equipped with an lmg, either the german or the soviet one.
+ pros:
+ firepower
+ price (you can merge them with conscripts and save 2mp per model

)
+ good performing anti-infantry nade
+ anti-tank nades (same you gotta upgrade for cons)
- cons:
- health; since they're using lmgs, try to use conscripts as a meatshield before them or stay on distance to not suddenly lose 3 guys in like 4 seconds. Make sure to stay away from enemy blobs, cuz they're gonna die quickly
- clumping; partisans come with 4 models, all quite close together, what makes them extremly vulnerable to nades and mortars (!) and tanks like P4 and Tiger
- veterancy; I am not entirely sure about this, but I guess in late game irregulars won't have such boosts as grenadiers or other german infantry forces do, what makes them worse then
- no call-ins; in this patch a lot of people just want to skip the expensive teching and call in something big instead, like the IS-2. Think this should be mentioned as well, but if you like teching and using the cons-repair for T34s or whatever: do it! =)
Note: at the moment irregulars are cheaper than Osttruppen - isn't that ridiculous?