A good rule of thumb is to use it on older copiers. The older the copier, the more time the generic toner manufacturers have had to perfect their product. The newer the copier is, the better to use OEM toner. Now there are exceptions to everything so if you want to change from OEM to generic buy it from the company that services your copier. That way if the change in toner messes up the copier you can hold them accountable. Plus, they probably know from experience which is the best generic brands to use.
If you are buying generic toner on your own be careful of buying on the internet because you won’t know what brand you are getting. Not all generic toners are equal. Some generic toner companies really have good quality control but not all. Plus, you should not mix generic brands. That could mess up your print quality. It is okay to put generic in if you’ve been using OEM but if you buy generic toners and don’t pay attention to the brands you could create problems. Again, you won’t have that problem if you are buying locally because they probably only buy one or two brands that their service department approves of.
In summary, if you have a print quality problem right after you add a new cartridge of toner it may be the toner and not the machine. Also, the advice above is for copier oriented MFPs. The HP world is another matter which I won’t go into now. There are different suggestions for that which I’ll address later.