Nobody takes apart old harddrive to "renew" them like replacing bearing. They are build in cleanroom and the only thing the "rebuild" does is sort them based on the amount of life left (reallocation, power on hour, etc) and put a new label on it before selling them. They are used with their own warranty, sort of like a used car with an aftermarket warranty.
If the price is right and you only need it for what you need it for, it is probably ok. My main concern is if you don't have any redundancy and you cannot afford the down time, it is not worth the savings. If you use it to archive and you have a lot of redundancy (like the cold off site storage I mentioned above), then it is worth using.
I wouldn't use it as my boot and application HDD on my desktop, not worth the savings.
