The PDW is probably the easiest weapon to use in BO2. Not necessarily the best, but the easiest: run toward enemy, fire in general direction of enemy, if enemy is not dead continue firing until either you or they have died. Rinse, repeat.
When you're going up against a lot of PDW users, it's really not a big deal. Same as always, it's not who can fire the most shots, it's who can get the hits faster...the PDW has an edge in having that huge magazine, but it's a low fire rate weapon with relatively low damage. The only scenario where it would be the de facto best weapon to have is when you've just flanked four or more oblivious enemies; in any other gunfight, it's going to be at a disadvantage against at least some other guns.
One nice thing about so many people carrying PDWs is that it makes it relatively easy for me to upgrade my secondary on my LMG & sniper classes. 