Interesting what happens to the model when the cheaper vent covers are used. With that said, I do have a cheap cover on my bathroom vent and the Ultra Breeze on the Fantastic Fan. Both are worth every penny for the protection they offer.
Oh yeah...worth it and easy to install. However, be sure you get the correct cover if you cover your High volume fantastic fan. You don't want to restrict the cover from opening completely and cause the fan to not move enough volume of air.
Fantastic Vent Corp is now producing a cover specifically for their power vents. I just installed two of them. While I don't like the way it mounts as well as the MaxAir II (which is the only MaxAir product I have installed), they allow you to open the vent more than the MaxAir and get better flow.