I'm a cheap you-know-what. BUT! I've seen too many studies (NOT funded by the drug companies) - illustrating pretty convincingly to me that - particularly with the ivermectin products... even though the generic may have a chemical analysis showing the same concentration of ivermectin as the name-brand Ivomec...efficacies are much poorer.
What good is it if you save a dollar (or $2 or $4...) per animal buying the cheap 'generic'... and it doesn't do anything when you give it at the recommended label dosage? What if you had to give 2 or 3X the label dosage to get the same effect... would it still be such a good deal?
Anymore, I only buy the original name-brand formulation... regardless of what class of dewormer it's in.
Pour-ons? Sure, they're easy to apply... but... they are possibly the least effective method of deworming. Even if you get it all on the cow, and none runs off onto the ground, it's really poorly absorbed - only about 30% of the applied dose actually gets where it needs to be to kill worms - and most of that gets there due to cows licking it off themselves and herdmates.
Using pour-on dewormers for fly control... is madness. I see folks recommending it here all the time... and some will say "I know it's probably not doing a very good job of deworming - but it gets the flies!" But... what it's also doing is selecting for a population of nematode parasites (worms) that are resistant to the class of anthelminthic drug in the pour-on.
If you ever need to use one of that group of drugs again for actual 'deworming'... it probably is not gonna work...
and what we're seeing with most of these parasites is that once you've selected for a population of worms that are resistant to the ivermectins &/or benzimidazoles (white wormers)... that resistance is there FOREVER. Resistance to levamisole seems to taper off over a 15-20 year period, and it may become useful on a premise again... but that appears not to be the case for the other two families of drugs.