When playing Battlefield 3, I use the PP-2000 as a primary weapon for all kits but support where I tend to chose a heavy machine gun. Even though I tend to use it at mid-to-close range I still have issues with the enemy killing me with non-PDW weapons. I'm at rank 25 so I would say I'm a confident player and have got a number of weapon unlocks for different kits, although it's difficult to chose between them all.
Though outclassed in most areas by other weapons, PDWs have the lowest hipfire spread (only rivaled by bullpup carbines equipped with laser sight), generally reload faster than other weapons, and allow the player to move at full speed (unless aiming down sight with mid-range scopes).
The majority of the PDW's (UMP-45, PDW-R and AS Val being exceptions) damage dropoff doesn't start until 12 meters in comparison to the usual 8 meters by most assault rifles and carbines, meaning that the PP-2000 and the M5K for instance will only need 4 shots to a torso to kill at 12 meters where a typical assault rifle would need 5.
Shoud be super effective in close range
PDWs are a great weapon from short-mid to medium range encounters. Great accuracy in the first couple of shots and an amazing rate of fire make these weapons pretty much unbeatable for fighting in close quarters or medium size city streets.
I personally like to use one due to the amount of speed you can maintain whilst firing, they also seem to ready faster than most weapons and fire accurately from the hip even after sprinting, making these weapons a great tool for rushing and game modes such as CTF or RUSH.