I just spent thepast 6-7 hours beating co-op and grinding for the unlocks with a friend. The unlocks are worth it. Especially if you play assault, the KH2002 and G3A3 might be among the best guns in the game. They're definitely on the same level as the AEK, which I considered the best gun before using these new unlocks. G3A3 close to mid range drops people better than any assault rifle while the KH2002 is probably the most accurate gun i've used; its still powerful as well, 1-2 bursts kills people and is effective in close range. Mind you I used these guns without any scopes or attachments and came to these conclusions. Just wait until I unlock all the attachments and scopes.
TL;DR: Grinding for the unlocks in coop is worth it. Especially for the burst pistol and Assault Rifles.
TL;DR: Grinding for the unlocks in coop is worth it. Especially for the burst pistol and Assault Rifles.