I got interested in upgrading my Fein vacuum to the HEPA filter when warned on FineHomebuilding's "Breaktime" that drywall dust would blow the motor. After that I found evidence some finer dust was getting through the standard issue filter.
I was tempted to buy a suggested "clone" HEPA cartridge for a lot less and use it along with the existing filter. Instead I trusted the guys that made this great vacuum and spent a lot of money for the little Fein HEPA cartridge (80$) filter (Fein really knows how to charge for their accessories), the accessory to hold it in, and 3 bags, but now I am happy with it. I went into it blind because I couldn't find any explanation of how it all worked. I was imagining a paper sack over the HEPA cartridge. It's a lot different than that.
The paper bag is quite a piece of work. It has a rubber rimmed opening that "snaps" over the intake "rib" and then the paper bag circles inside the vacuum tank - circling the HEPA filter cartridge. My understanding is that the HEPA cartridge is supposed to only be a "last check" on fine dust.
The vacuum doesn't seem to have diminished in suction. I AM going to try to empty and reuse these bags - blow them out with air and roll-staple them with a bag stapler. Maybe also tape them with Gorilla tape. So I am cheap but happy with the system so far. It all seems to have the quality built in.