I have used several types of bags by this company for several years (four types - from small to large) and they seem to work just as well as the makers' bags. They aren't exactly the same but they fit and work perfectly.
One advantage of cheaper bags is that can afford to change them more regularly.
Also, with some non-reusable paper bags you can always open the bottom of the bag, empty it and either staple or clip the bottom back together for a second or even third use (provided the pores aren't so blocked that it prevents the air passing through easily).
I far prefer paper bags (even re-used once or twice) to the permanent cloth bags you can sometimes buy to replace them. In my experience cloth bags do not filter as well as paper bags.
As with all vacuum cleaners, always check the airways and permanent filters from time to time. I have often seen vacuum cleaners so blocked that it is obvious that they have been redistributing rather than cleaning for some time (especially when in communal use).