High end KO's - really awesome (and dangerous) such as the KO japanese headmaster set. very hard to tell without close inspection!!
"Supposedly" high end KO's - can be pretty sh*tty sometimes like the wheeljack. you will never confuse this guy with the real deal! this range is a hit and miss.
Cheap Chinese KO's (typically carded) - some are just nasty. others are solidly built and really great for custom projects when you need spare parts or if you dont want to chop up that genuine $150 etc...
Unique KO's - oversized stuff can be pretty enjoyable due to the novelty. (like my two Bruticus'... oh yeah!!!)
Get the high end G1 KO's if you already have the original and want to educate yourself on how to identify the fakes. nothing beats hands on experience.
However, dont buy KO's if you are desperately wanting/needing that figure for your collection, as you will be unsatisfied when you get it due to that nagging feeling and the potential chance of getting something that is poor quality. Only buy it if you really dont mind it and will enjoy it.
Its entirely up to you, but i would try to hunt for genuine G1 versions first as they are more rewarding to own.
If you only want minty G1 figures then you must save up for them or try harder to find the bargains. you will be much more satisfied when you put the effort in.