Okay, everyones right! And, everyones wrong!
Lings rock. <--- FULL STOP!!
They are not invicible. <---- FULL STOP!!
If you just mass lings you will be snapped by: Reavers, Archons, Zealots (sorta), High templar, dark temlar (if the overlords are taken out), tanks (with close defense), bats+medics, marines + medics, bats/marines in bunkers, irradiate (possibly), BC's, lurkers, guardians, *MORE* lings
AND alot of things i probably forgot - hey, its been like a year since i played SC:BW (now onto WC3).
PS: lings rule!